B: Pond keepers A to Z … Bacteria to Buying Koi

B Bacteria (Plural of bacterium)

We can’t see them, smell them or hear them yet their existence is fundamental to successful fishkeeping. If it wasn’t for these microscopic single-celled organisms, koi keeping would not exist (We wouldn’t exist either!) as they have such vital roles to play in the koi environment, breaking down toxic pollutants but also causing disease.. Bacteria come in many different shapes and sizes, some have the ability to move and swim about while others can breakdown and digest some of the most resistant materials.

Goodies: These largely account for the ‘bio’ part of the biofilter and are responsible for the breakdown of lethal toxic waste products excreted by fish, particularly ammonia into nitrite which is still toxic and then into nitrate. The biofilter is the heart and lungs of a pond system, full of helpful bacteria, these invisible lifesavers are easily killed if the filter is turned off, through a combination of drying, starvation and suffocation.

Tip. Try to keep your filter running constantly and never clean out the whole filter in one go as this will harm the thriving bacteria population that is working hard to keep your pond water ‘sweet’. Essentially there are two categories of bacteria involved in the breakdown of waste in a koi pond system. Those that breakdown inorganic compounds and those that digest organic compounds.

Firstly, those that are involved in breaking down and detoxifying ammonia into nitrite (Nitrosomonas bacteria) and nitrite into nitrate (Nitrobacter). These are aerobic bacteria and must be provided with oxygen. In fact the more oxygen that is provided the better they work. A trickle filter is more effective than a submerged ‘wet’ filter as it is standing in 21% oxygen in the air compared with 1% DO in water.

Nitrates can be reduced to nitrogen gas in anaerobic conditions where anaerobic bacteria take their oxygen from the nitrates to form bubbles of nitrogen gas.

The other group of bacteria is less well known in pond keeping circles but they too have a vital role. They are involved in breaking down all organic compounds in a pond or filter system. Organic compounds could include solid wastes such as faeces and other detritus or dissolved substances such as amino acids and sugars. They are like you and me in that they require a food source which contains carbon. If these bacteria did not perform their vital role then filters would block up more quickly and settlement chambers would contain more solid material.


Pathogenic bacteria cause disease in fish, causing an array of symptoms including ulcers, fin rot, and mouth ‘fungus’ which despite its name is actually caused by bacteria.

These baddie bacteria are opportunistic in that they are constantly found in the water in low numbers, waiting for an opportunity to infect a fish that is stressed or wounded. Once they have found a suitable environment, they will divide and multiply rapidly digesting fish tissue, causing distress through the toxins which they produce and the physical damage and stress response they cause. External bacterial infections can be treated using malachite green or other antibacterial solutions or internally through antibiotic treatments using medicated food or injections.

Recent issues have arisen relating to the treatment of bacterial problems where due to the occurrence of resistant bacteria, some diseases have become difficult to treat.


Also often spelt ‘barbules’ these are one of the koi’s organ of taste. Koi are far more at home rooting around in the soft clay substrate of a natural lake, using their barbels to feel and taste a potential morsel on which to feed.

As koi keepers largely feed floating pellets, the acute sense of smell and touch associated with the barbels can be used to locate their food on the surface. Koi have two pairs of barbels, a tiny set on the upper lip and two larger ones at each corner of the mouth. Being covered in taste receptors, barbels allow koi to taste food before having to ingest it. Quite handy if you’re a shy or suspicious feeder, which is typical in wild or undomesticated carp.


The British Koi Keeper’s Society, like many other national koi keeping clubs and societies throughout the world act to promote, educate and support koi keepers. The BKKS is based around a network of local sections where hobbyists meet periodically to discuss koi-keeping issues, hear visiting speakers, visit other koi ponds or arrange their annual section show. If you want to get more involved in koi keeping then contact the BKKS direct for your nearest local section or look out for their fliers in your local koi or aquatic centres.


A quality koi must exhibit the desired features in a range of criteria if it is to win an award at a show. Some of these criteria include size, colouration, deportment and bodyshape. Bodyshape is significantly different between the sexes in mature fish, with females being larger, deeper and more robust in shape than their slender more torpedo-like males. To gain high marks for bodyshape when being judged in a show, features such as the proportion of the head, fins and tail in relation to the rest of the body are important as is the volume of the whole body. The koi should not be too thin either or resemble a rugby ball. Changing feeding regimes or stocking densities can alter bodyshape and many successful koi keepers have their own tricks of the trade for gaining good volume while maintaining good overall proportion.


Bonsai is the ancient Japanese art of growing trees in a confined space. Bonsai can be translated as ‘pot’ and the aim of Bonsai is to recreate some of nature’s most stunning effects on trees that are reduced in scale (an usually grown in a pot). What’s all this got to do with koi keeping? Many koi ponds are set in themed Japanese settings or landscaped gardens, where decking, gravel, granite statues, lanterns and Bonsai are placed to put the koi in full context.


Perhaps the aim of many koi keepers is to breed from their own fish. Breeding koi in a pond can often turn into a mixed blessing. Firstly it is encouraging for koi to breed in your pond as it is an indication that you have provided excellent water conditions consistently for many months. However, breeding activity can often lead to damage, especially in females as a result of being chased and driven by the amorous and physical attentions of the males.

Koi produce many thousands of translucent, adhesive eggs that are fertilised externally by milt released into the stream of eggs by the males

Did you know? Koi eggs must be fertilised by a sperm within 60 seconds of them being released into the pond water, otherwise they will swell and be unreceptive to sperm. It is because of the long odds of fertilisation actually taking place that koi release so many eggs.

Naturally, koi fertility rates are very low and as a result, intensification of koi farming techniques now allows on average 80-90% fertility rates through hand stripping and artificial fertilisation techniques.

Even so, the odds are stacked against us to produce quality offspring from our koi as most juveniles are ‘throw backs’ being poorly coloured and not resembling either parent. Usually, single figure percentages reach the market with line bred broodstock in Japan – (And that’s with generations of experience and selection!)

Brocaded carp

The literal translation of Nishikigoi. Typically given as a description for a fabric with a raised design, brocade has now also been used in the description of the myriad of coloured variation, literally being described as brocaded coloured carp.


Brineshrimp are a vital food for large-scale intensive culture of koi fry. When koi fry hatch, they will spend 2-3 days absorbing their yolk sac, after which they are ready for their first food. If stocked out into a mud pond at this stage, a living soup rich in microscopic zooplankton such as rotifers is essential for the survival of these fry.

However, a rotifer bloom is not essential if the fry are kept indoors in high-density recirculation systems. For their first 2-3 weeks they can be fed quite happily on newly-hatched brineshrimp until they are strong enough and developed enough to be weaned onto a high protein powdered diet.

Brineshrimp are imported into the UK from USA salt lakes where they are a natural harvest, forming cysts within which they can survive long periods of dry weather. They can be brought out of this state of suspended animation by re-hydrating them in illuminated, aerated and saline water at 25 degrees C. Within 24 hours they will have hatched and be ready for feeding to the koi fry. To achieve effective growth, this should be done 4 times a day on a rota system.

It is easy to see whether the fry have eaten or not as they should have a thin orange stripe along their body, showing that their gut is full of brineshrimp.


A flexible rubber material very popular for lining koi ponds. Typically 0.75mm thick, a butyl rubber liner can offer up to 50 years of life and is easy to install. Its flexible nature means that it will stretch under the substantial pressure of water, making it less likely to tear than other liners. It has become less popular over the last decade as the strength and reputation of the less-expensive PVC liners has improved, now being offered with substantial guarantees.

Buying koi.

We all have to do it, some more successfully than others. Buying koi is not just a matter of choosing healthy attractive fish, but also paying the correct price for them. How much more do we enjoy those koi when we can also remember how little we paid for them.

What to look for when buying koi.

Choosing healthy fish that show good promise are likely to reward you with good growth and development for years to come.

Koi selection check-list

* Healthy fish are active fish and will not hide or sulk in the corner of the stock tank. If they show an interest in the prospective purchaser then this is a good indication of health.

* Diseased fish should never be on sale. However, it is always best to check and look for signs of disease such as ragged fins, imperfection or abrasions on the skin and flicking and scraping fish irritated by parasites.

* Ask the shop owner how long the fish have been in the shop. It is best to allow a week or so between importation and purchasing as the fish are likely to be stressed and very susceptible to disease. If possible, buy quarantined fish.

Jargon Busters

Organic: A substance that contains carbon and is the product of life. Inorganic: A substance that does not contain carbon and is not the product of life.

Trickle Filter: A filter where the media is in effect standing in air and water is trickled down through it (eg a top-fed box biofilter), making it very effective as the bacteria have a rich supply of oxygen.

Wet Filter: A filter where the media is submerged in water (eg an undergravel filter). Rotifers: Microscopic ‘bugs’ that feed on dissolved and microscopic organic particles. Also known as infusoria.

Kill blanketweed and string algae.