Pond fish food to filters

When I was discussing this article I suggested a ‘How it works’ series, covering anything from food right through to filters. My offer was misunderstood and I was taken quite literally at my word (and was asked to write ‘How it works – from food to filter’). But then I thought, ‘how novel – what a challenge’, and yes – there is a story here – with a beginning, a middle and an end – so hear it is.

Beginning. Food:

Pond fish are just like any other fish or animal in that they have particular nutritional requirements. After water quality, their nutrition comes a very close second to having the greatest influence on the their health and appearance.

A fish’s diet will affect its growth, coloration, resistance to disease and its breeding performance. Any deviation away from its nutritional requirements will not only be your customers’ responsibility, but also to their ultimate cost and dissatisfaction with inevitable downturns in health, growth and color.

The artificial pond fish diet.

Most pond fish are omnivorous, feeding naturally on items that are either animal or vegetable in origin. These inquisitive fish constantly scavenge over a natural pond bottom feeding throughout the day, making the water muddy as they dig around. Readily consumed items include worms, insect larvae, algae, plant roots, shoots and other detritus (both dead and alive) that may have settled on the pond bottom.

But as pond fish are traditionally kept in a densely stocked pond, they are completely dependent on the pondkeeper to provide them with a suitable artificial diet. It is not practical or economically viable for the pondkeeper to feed their fish on naturally occurring foods such as daphnia, bloodworm and fresh vegetable matter.  Problems would arise with the season availability of say daphnia, which would also need to be collected on a regular basis (if some could be found). However, because of many years of nutritional research across the world, it is possible for pond fish to obtain a complete and balanced diet from a dry artificial food.

Complete and Balanced.

Complete and balanced mean two different things when describing a diet. If I went to a fast-food burger bar and ordered a quarter pounder with all the trimmings – then I could argue that this was a complete food. That is, it contains something of all the nutrients that we require to remain healthy. However, the bad press that fast-food restaurants have been receiving recently is due to the food being unbalanced. That is, even though all the nutrients may be represented in their food, they are not in the correct ratios or quantities. So that if you consumed nothing but that diet, you would experience health problems on account of its unbalanced nature. The same is true for our koi. If we endeavour to locate a complete and balanced diet then they will thrive. If we don’t, then in essence we are guilty of sending them to a fast-food restaurant for every meal – with the very predictable consequences.


How do fish use the food we feed?

The 3 key areas- Health, Growth and Color.

1. Health.

A specific factor in a diet that will affect health is vitamin content.  These very ‘fragile’ compounds, although found in minute quantities in the diet, are essential for the healthy functioning of a myriad of biological processes.  Often excessive temperatures that food undergoes during its manufacture can cause some vitamins to be lost from the natural ingredients.  (Similar to over-boiling vegetables!).

Fish nutritionists have recognised this and advocate the inclusions of supplements of stabilised vitamins, that remain unaltered and intact through the manufacture, available for fish.

Vitamin C is very prone to being lost under such circumstances and Stabilised Vitamin C can now be found in most leading diets, to enhance the fish’s immune response, preventing deficiencies as occurred in sailors of old, who had diets deficient in Vitamin C.

2. Growth

The most cherished and memorable fish in a collection are the larger fish.  In pond keeping, size certainly does matter, and it is the aim of many pond keepers to grow their fish as big as possible (in as short a time as possible).  As most fish are sold on size, the perception of value in a fish can be largely determined by its size.  Several factors interact to control growth rate, some of which we can and can’t control.  Genetic factors that influence growth rate are fixed, and are a factor of inheritance.  Growth rate can be manipulated by keeping the stocking density in a pond under control.  For example, fewer fish in a larger pond will soon outgrow a heavily stocked, but smaller pond. 

The food that is offered to pond fish can also be controlled, in both its quality and its quantity.  Fish food provides the energy for movement as well as the protein for growth.

Protein in the diet can be both an enemy and an ally, and care must be taken when choosing a food for you pond fish.  All fish foods are different, and the role of proteins must be fully understood to be able to make the wisest food selection.

- What are the uses of Protein?

Fish can use protein in 2 ways, either for growth (which is preferable) or as an energy source (which is not).  If fish use the protein they consume in their diet for growth then they will increase in length and weight.  But if pond fish use protein from the diet as a source of energy, (instead of other energy providers such as carbohydrates or oils), then growth is likely to be reduced and when protein is ‘burnt off’, toxic ammonia is released into the water as a by-product.  We can encourage pond fish to use protein efficiently for growth by choosing a balanced diet, appropriate to the pond temperature.

- Water temperature and protein.

Spring and fall 

In cooler waters, either side of autumn and spring, it makes sense to feed lower protein diets.  Fish are not growing sufficiently fast enough to justify a high protein diet (which is more expensive), and if they were fed such a diet, they would ‘burn off’ a lot of the excess protein for energy, and release toxic ammonia as a by-product.

In addition, at temperatures between 50oF + 60oF, digestion is relatively inefficient in pond fish.  It is for this reason it is wise that fish are fed a low protein vegetable based (wheatgerm) diet which is easy to digest.


As water temperatures begin to regularly exceed 60oF, and fish become more active, they show a greater potential for growth, if offered sufficient protein in the diet.  For this reason, pond fish should be offered a high protein diet in the summer to optimise growth in pond fish.  If they were maintained on the low protein wheatgerm diet over this period, then the lower protein in the diet would not enable fish to grow as quickly.

3. Color.

Pond fish are bred for their array of colors and the visual impact they have when assembled in a pond.  Color can be compared to growth, in that it too is a factor of a fish’s genetic inheritance, but can also be improved through nutrition.  It is possible to enhance and improve the color of pond fish by feeding color-enhancing compounds in the diet.  Pond foods can now be formulated to include a range of natural color enhancing ingredients such as Spirulina (an algae), Krill (a marine crustacean), and even Paprika (red pepper) or marigold petals.  These ingredients contain pigments called carotenoids which are the compounds that ornamental fish such as koi and goldfish exhibit in their skin.  These same fish can store more pigment in the skin if it is made available to them in their diet, deepening the red in their skin.  Look out for color enhancing foods, so you can play a part in improving the color of your fish.

Box out: How can such a dry, visually unappealing and ‘dull diet be so popular and successful at rearing fish worldwide?


1) Flexibility in diet formulation.

Pelleted diets can be formulated into specific diets for different species.

E.g. Protein source and content, lipids, color enhancers, medication.


2) Predictable formulation

Unlike a natural diet that varies seasonally, a complete diet of consistent quality can be offered all year round.


3) Digestive improvements

Manufactured diets are ‘milled’ into an easily digestible blend of products, which complement each other. Similar to processed food in our diet.


4) Pellet qualities

a) Floats – enables inspection of fish

b) Prevents overfeeding – remaining as discrete pellets thereby reducing pollution.

c) Size – manufactured to suit mouth-size (powder to large pellets).


5) Long Shelf Life

Pellets are dry to prolong shelf-life by reducing bacterial breakdown. Can contain antioxidants and mould inhibitors. Moisture should be as close to 6% to inhibit microbial breakdown.


6) Easily Handled

Can be bagged, scooped, put into an autofeeder. Makes world-wide transport relatively easy.

Middle: Inside the fish:

When pondfish feed, their objective is to digest the food they consume efficiently, absorbing what they require, excreting what is indigestible or surplus to their requirements. Digestion is the breakdown of insoluble food items into less complex, soluble molecules that koi can then absorb into their blood. This involves both mechanical (physical) and chemical stages, and the feeding process starts before food is offered to them as fish are likely to detect you approaching the pond with food.

Pond fish are alerted to the presence of food through a combination of sight, sound and smell, as they either see or hear food (or other feeding koi), or smell the scent of food that has dissolved in the water.

On discovering food, its mouth and buccal cavity (at the back of the mouth) are well equipped for the seizure, control, selection and preparation of food.

The buccal cavity is lined with tough ridges of a folded membrane called the mucosa. The mucosa is covered with microscopic projections called papillae and is richly provided with mucus-secreting goblet cells and taste buds.

Chewing it over

Digestion in humans starts in the mouth with chewing to physically break up the food. This is also true for ornamental carp where preparation of the food commences with the grinding action of several pairs of pharyngeal teeth. They are situated in the posterior part of the buccal cavity on the ventral surface. This “grinding mill” ensures that all food entering the gut is suitably fragmented for further digestion. The “chomping” action seen in a feeding koi and goldfish occurs as a result of this grinding, often expelling any rejected fragmented material through its opercula.

The duct which carries the ground-up food from the pharyngeal region of the buccal cavity to the anterior intestine and receptive sac is called the oesophagus. A dense bed of cilia and goblet cells (secreting yet more mucus) aids the passage of the selected food material through this first constriction in the gut. Fish consume many insoluble organic nutrients in their diet, such as proteins, carbohydrates and oils with complex molecular structures and high molecular weights.  To utilise them, they have to be broken down into smaller component parts which are soluble in water and easily transported around the body to where it is needed in the fish’s blood system.

Breaking It Down

A koi’s intestine is a relatively featureless structure and unlike other vertebrates, has no multi-cellular secreting glands, valves or caecae. In fact, apart from the dilating receptive sac (which is not a stomach), the intestine is similar in structure along its entire length. It is usually twice the length of the fish’s body and is lined with epithelial cells which are constantly being replenished as abrasive food material erodes them away. The passage of food is lubricated by secretions from goblet cells while other cells in the gut epithelium are dedicated to absorbing the broken down soluble nutrients from the gut.

Carp – The stomachless fish.

The grinding action of the pharyngeal teeth may remove the necessity for the action of acid on food material. The complete absence of a stomach means that unlike other fish, no acid digestion takes place and no pepsin (a protein-digesting enzyme) is produced in conjunction with hydrochloric acid to digest proteins. In those animals that do have a stomach, pepsin acts in the stomach optimally at pH2, so if no hydrochloric acid is secreted in stomachless koi then pepsin secretion would be pointless anyway.

It has also been suggested that due to the omnivorous diet of koi compared with other fish, the need for pepsin is reduced as lower levels of protein are ingested in their diet. This is probably why protein assimilation efficiency tails off when koi feed on very high protein artificial diets.

When comparing the intestines of fish it is interesting to note that those of omnivorous fish such as koi are always longer than those of carnivorous fish. Koi diets are lower in protein and are harder to digest, requiring a longer time in the gut to be broken down and absorbed – hence their lengthier guts.

Detergent-style action

Ducts from the gall bladder and pancreas enter the koi’s intestine almost immediately after the oesophagus, carrying bile and enzymes respectively into the intestine. The food and enzyme mixture remains in the distended receptive sac while it digests, prior to entering the smaller intestine.

Bile which is produced in the liver and stored in the gallbladder is an emulsifier of lipids and oils. Working in a similar way to a detergent it breaks up larger bodies of lipid into smaller droplets producing a larger surface area for lipase (lipid-digesting enzymes) attack. Lipids and oils are broken down into soluble fatty acids and glycerol so that they can then be absorbed into the blood.

Proteases such as trypsin and chymotrypsin, secreted by the pancreas, digest the proteins in the food. This breaks down proteins into amino acids or small groups of amino acids which are soluble and easily absorbed into the bloodstream.

In human digestion, the action of chewing, smelling and seeing food stimulates the mouth to “water” or produce saliva. The saliva contains a starch digesting enzyme (salivary amylase) which breaks down insoluble starches and  carbohydrates into soluble sugars. (Try sucking on a piece of bread (starch) and after a while you’ll find it tastes quite sweet). In its aquatic environment, it is impractical for koi to secrete salivary amylase, so it is secreted directly into the gut from the intestinal mucosa to break down starch.

How long does food spend in the gut?

 Gut transit time (the time taken for food to pass through the fish’s intestine) can range in koi from 16 hours at 25°C up to 60 hours at 12°C, showing that rates of digestion and enzyme action increase along with the temperature. Prior to excretion, the final absorption takes place in the hind area of the intestine. Further towards the anus, the intestine is similar in structure to the oesophagus, showing thicker walls of connective tissue and being densely supplied with mucus secreting goblet cells. This aids the formation, passage and ejection of waste. In the natural carp diet, a significant percentage of faecal material is inert mineral matter of no nutritative value, inadvertently ingested with food items while grubbing around on the pond bottom. Unlike carnivorous fish, the carp does not secrete collagenase, an enzyme required for the complete breakdown of the connective protein found in animal skin and tissue and this will also be a significant part of faecal matter when feeding on natural diets.

When comparing the diets that we offer our fish with the natural diet that they are designed to digest, we could be guilty of offering them a diet that is too easy for them to digest. It is accepted that low-waste diets lead to better water quality, but are adapted from high performance diets that are used to grow food fish to a harvestable size as quickly as possible. Would it not make sense to offer our koi the benefits of a more challenging, roughage-rich diet, acknowledging that we too are keen on growing our koi quickly, but that their long term health is also of paramount importance to us? Having seen how their digestive system is adapted for a tough and varied diet, can this really be achieved by feeding koi a diet that is far removed from what nature intended?

End – The filter

Pond Filtration:

All living animals produce waste which is toxic and must be excreted.  If such waste is allowed to build up once excreted, then it is likely that health problems will follow. This is true for pond fish just as it is for slums in city areas of developing countries. Fishkeepers have learnt from the valuable experience gained by our ancestors who invented and installed sewerage systems and later on treatment works to manage the problems of domestic waste. These principles water treatment are applied very well to fish keeping and especially in the filtration of garden ponds.  Without filtration, fish keeping and pond keeping would experience many limitations.

Why do ponds need filtration?


In natural balanced aquatic ecosystems, such as oceans, rivers or lakes, fish are in balance with their surroundings.  There is sufficient food on which to feed, and they are so lightly socked in relation to the water volume that there is no build up of pollution.  Their natural environment is self- sustaining.  This is not true in most garden ponds.  Ponds are typically well stocked with fish in all shapes, colors and sizes, well above the natural stocking density.  So much so that they require food to sustain them (as the  pond will not provide sufficient naturally), and in a similar way, there is insufficient natural bacterial action in the ponds to breakdown the waste produced.  In most cases, no filter – no fish.

How does a pond filter work?

Pond filtration can be broken down into essentially 2 functions: 


Function 1- Solids Removal


A filter’s first function is to remove solids.  Mechanical filtration traps solids that are carried suspended in the pond water.  Solids can be removed in a number of ways, depending on the size and design of the filter.

1).  Entrapment.          Acting in a similar way to a sieve, the first filter media that the pumped dirty water encounters is quite coarse in structure, trapping and removing suspended solids from the water.  This is the method used in standard external black box biofilters, where 2-3 grades of foam, running from coarse through to fine act to trap solids as they pass. 

2).  Settlement.           If the speed of water flow is reduced, then its ability to keep solids in suspension is also reduced, resulting in the solids becoming settled out of suspension.  This method of removing solids is used in larger, multi-chambered filter systems, where the pipework removing dirty water from the pond opens out into a vast settlement chamber.  Compared with the fast flow rate of water through the pipe leading from the pond, the water in the first chamber slows down so much that solids drop out of suspension.  In many such systems a second settlement chamber is fed from the first through “up and over” weir boards, slowing the flow down even more, helping to settle out the finer solid material.

Settlement as a means of mechanical filtration is best fed by gravity rather than pumped water.  Water that leaves the pond via a bottom drain is far better at handling and removing solids than a pump.  Pumps are usually fitted with a foam or perforated plastic guard to prevent debris from choking the impellor. Unfortunately, this can also prevent leaves and other larger solid particles from reaching the filter, being retained in the pond, causing the water to cloud or silt to build up on the pond bottom.

More recently, pumps specifically engineered to handle solids have become available which can overcome this problem.  However, such pumps can have a tendency to ‘liquidize’ larger particles into finer material, making it harder and less likely to settle out in a filter.

Is clear water always healthy?

Many pond keepers are happy to see their pond clear of solid waste, taking the view that a clear and debris-free pond is a healthy pond.  Paradoxically, Koi, Orfe, goldfish and other pond fish prefer the water to be cloudy and rather green, thriving in such conditions in clay-based fish farm ponds.  Solid waste or suspended matter will barely affect fish health, but the real fish killer is the colorless and invisible ammonia excreted by fish.  It is impossible to tell whether clear pond water is safe and healthy just by looking at it.  However, if it is filtered continuously through a bio-filter, then it is very likely to be healthy, and this is the second function of filtration.

Function 2 – Bio-filtration

Having safely removed any solids from the pond water, the clear water is now passed through the part of the filter specifically designed for bio-filtration.  A bio-filter, as perhaps can be gleaned from its name, is a living filter, colonised by many millions of bacteria whose role is to consume and breakdown the toxic ammonia that is constantly being excreted by fish (and other aquatic organisms).

These beneficial bacteria will colonise any hard surface (including the pond liner, pipework and rockwork).  However, a bio-filter is designed to provide a vast surface area on which these bacteria can colonise, providing the surface area in a filter which may naturally be found in many square metres of a natural pond or lake bottom.

Keeping a filter alive.

In addition, these well-housed bacteria are provided with a luxury lifestyle, receiving all their requirements for a long and healthy life.  The steady turnover of water through the filter provides a constant source of ‘food’ – in the form of ammonia, as well as an essential supply of dissolved oxygen.  It is recommended that the pond volume is turned over at least once every 2 hours. Some bio-filters are fitted with airstones to ensure that these aerobic bacteria receive sufficient dissolved oxygen.  In a balanced pond, the filter bacteria should be able to break down the ammonia at the same rate at which it is released by fish. 

As this vital part of filtration is ‘living’, unlike mechanical filtration, the bacterial colony takes time to become established or ‘mature’.  It is for this reason that a pond must be run-in gently over the first months of its life.  Fish should be added a few at a time, so that the bacteria can adjust and catch up with the rate of ammonia being produced by the fish.  If too many fish are added too quickly, then ammonia levels will rise rapidly, causing fish stress and ultimately leading to disease. 

Ammonia is produced by fish as a result of them breaking down protein in the diet.  The more food that is offered to the fish, the greater the levels of excretion, and the harder a bio-filter has to work.  This is the reason why overfeeding kills fish, and is why water quality problems occur far more frequently in hot summer weather, when fish are feeding at their greatest. 

Watch out for nitrite as well as ammonia!

Bio-filtration performs a further key role that is life-saving.  Aerobic (oxygen-loving) bacteria breakdown the toxic ammonia into nitrite, which unfortunately, is still toxic.  In fact nitrite has a nasty habit of being more difficult for a different set of bacteria to break down.  It will persist longer than ammonia in water that is suffering a quality problem.    The bacteria responsible for breaking nitrite down into relatively harmless nitrate thrive in the same conditions as the ammonia-loving bacteria, which is a steady flow of well-aerated water. 

The fact that nitrite lingers in the water longer than ammonia means that it is a useful indicator as to the maturity and capacity of the filter.  The water can be tested very easily using a test kit for nitrite, and should be done so as a useful guide  as to the maturity of the filter.  The only desirable nitrite reading is zero and a positive reading shows that there are too many fish and too much food in the pond relative to the bio-filter’s capacity to break it down.  The course of action in this case is to stop feeding, carry out a 30% water change to dilute the toxic nitrite and only resume feeding when the nitrite reading has turned to zero.

Biofilter Design.

A bio-filter is fitted with media that is suited to being colonised by bacteria.  Essential features include a large surface area, a structure that is quite fine and will resist clogging but is easy to clean.  The media must also be inert, in that is does not interact with the water quality.  Historically, gravel has been the fishkeeping filter media standard, but more recently, lighter materials that offer a larger surface area have become more popular.  These include foam, various types of matting, sintered glass, and perforated or reticulated pipe.

Clearing green water.

Pond filters act to remove solids and detoxify the water, but they will not reliably clear green water.  The microscopic  algae cells that bloom in pond water to create green water are far too fine settle or be filtered out mechanically. A guaranteed method of clearing green water is to install an ultraviolet clarifier (UVC) between a pump and the filter.  The radiation emitted by the UVC causes the microscopic algal cells to clump together in green sticky ‘flocks’, which are of a suitable size to be removed mechanically by a filter.  UV’s consistently produce significant quantities of solid matter which can push a box bio-filter to its limit.  It is useful if using a UVC to opt for an oversized filter that will cope with the excess solid waste.

Care of the filter


Filters must be treated like a living entity. If they are not provided with oxygen, and food, they will deteriorate and die.  For this reason, a bio-filter must be run continuously, ensuring that the bacteria are provided with the materials for life.

There are times, as with any filter, that it must be cleaned and maintained.  In the summer especially, waste will build up rapidly in a settlement chamber or within foams, and these must be cleaned out before they clog or restrict the filter too much.  This can be done without disturbing the more sensitive bio-filter. 

If  cleaning out a filter chamber, the action of simply opening a valve may be sufficient to purge the chamber.  In a box filter, where the foam layers may act as both mechanical and biological media, care must be taken when rinsing out the foams.

Bacteria are very sensitive to changes in their environment and any adverse action could set the filter’s maturity and efficiency back months.  For this reason, when rinsing out the foams or cleaning any biological media, buckets of pond water should be used.  If raw tap water is used, then chlorine and other deficiencies in water quality can have a detrimental effect on the bacterial.

Pond Filter Designs: Advantages and Disadvantages.

Box filter

Advantages:                  1.  Cheap, and easy to install, ideal for a novice.

                                    2.  Quite effective in small/medium ponds

                                    3.  Can be installed in an existing pond.

Disadvantages:             1.  Difficult to hide, as they are best placed above the

 water level

                                    2.  Do not usually have separate mechanical and

                                                 biological areas which can cause difficulties

 during maintenance.

                                    3.  Must be fed via a submersible pump, which to be

 efficient, must handle solids.

Gravity-fed multi-chamber filter

Advantages:                  1.  Its large size and discrete chambers make it a very

 efficient filter.

                                    2.  Gravity fed from a bottom drain, no risk of

 overflowing, and can handle solids very well.

Disadvantages:             1.  Relatively expensive to install, taking up a lot of

                                                 room (although it is below ground).

                                    2.  Must be installed when pond is constructed

 (generally not for the novice).

Kill blanketweed and string algae.