Summer Koi Pond Care.


“Summer time, and the living is easy,

          Fish are jumping and the cotton is high…..”

This is what we’ve all been waiting for.  Preparing our koi pond through any winter worries, ensuring that the koi have come through their ‘rest period’ free from any health problems.  Spring has seen any aquatic plants that you may have in your koi pond re-potted or trimmed if necessary and the new shoots and growth seem to be emerging from all directions.

The hard work done, the stage is set and the curtain is about to be raised on the splendour and vitality of the summer show.  As summer approaches and the temperature rises each week, the pond takes on a new life of its own, as though the heartbeat of this dormant creature is about to increase as it springs into life.

The pond water will start to behave differently in these months of summer excess.  The warmer weather can push oxygen to the limit, causing koi a number of problems.  Various precautions should be taken to avoid stressing the pond’s inhabitants, and an eye kept on various water parameters.

Your koi will also enter a different stage of their calendar of development, with a heightened appetite for food, as well as each other, as the time for spawning approaches.

Even though the ‘living’ should be easy in the summer time, and the pond should be thoroughly enjoyed on a daily basis, there are a number of key areas to keep a check on to make sure that your pond and koi have a successful journey through summer.  The bet advice is “be vigilant”, as things can grow and even progress out of hand at an alarming rate through summer.


As your koi are cold blooded, their behaviour, appetite and growth rate is largely determined by the water temperature.  They would have started feeding in spring, once pond temperatures had consistently exceeded 10oC, with their appetite improving with each rise in temperature.

Summer is a vital few months for your koi, as it is a relatively short window of opportunity for them to perform some essential tasks.

1.         Koi must eat to satisfy their daily energy requirements to grow, and in addition, consume sufficient food to deposit their winter reserves.

As temperatures rise above 14oC, koi should be offered higher protein ‘growth diets’ that will fuel daily activity and growth, but also supply quality nutrition that can be stored for the autumn and winter months.  If your koi receive sub-standard nutrition then it will not only affect the health and growth of your koi immediately, but also threaten their health and survival over the winter.

2.         When you feed your koi, you should also remember that you are indirectly feeding your pond.  It is not the end of the matter once koi have consumed their pellets (or fresh treats), as the water will be burdened by excreted by products produced by your koi.  In summer, when fish are feeding at their highest, waste production will also be at its highest level and this may cause water quality problems.

Filter it out.

The waste will take two forms, and must be dealt with effectively by your filter.  A lightly stocked ‘natural’ pond that is not filtered must be fed with great care to prevent the water from deteriorating.  As these are in the minority of koi keepers, we rely on a pump and filter.

-  Solid waste must not be allowed to settle on the pond bottom where it will decompose, but should be pumped out of the pond into a filter.  This will reduce the likelihood of the water becoming turbid as solid material will not be stirred up by koi.

-  Soluble waste.  The most toxic waste excreted by your koi is ammonia, which is soluble and colourless and excreted through the gills.  As it can’t be seen, it can often be overlooked as a hazard to your koi.  However, koi produce it at the same rate at which they are fed.

A biological filter is required to break down ammonia into less toxic nitrite and then nitrates.  Before increasing the feeding rate during the summer, it is essential to check that your pond is adequately filtered.  Otherwise, your koi may not just be eating you out of house or home, but also themselves into health problems.


As koi activity and appetites increase with temperature, so do their demands for oxygen.  Unfortunately for koi, as the temperature increases, and their oxygen demand increases, the oxygen levels within a pond will decrease.  This is because warmer water will hold less oxygen than cooler water. (Watch bubbles form on a kettle element when it is switched on).

To provide enough oxygen for your koi, there are several courses of action that can be taken;

1.         A waterfall or fountain.  Either of these water features will disturb the water’s surface and mix in extra oxygen, alleviating any oxygen deficiencies.  Be careful to keep the intake of the pond pump clear of algae or debris as a reduction in turnover or flow rate down a water fall or fountain will also have a knock-on effect on the dissolved oxygen levels.

2.         Install an air pump with diffuser.  Pond air pumps can be easily installed and fitted with an airstone at the bottom of the pond or filter chambers to increase dissolved oxygen and water circulation.

3.         For those with plants in the pond, keep at least 50% of the water surface clear of plant cover.  Too many plants covering the surface of the pond prevent diffusion of air into the pond.  Ponds that are home to duckweed or fairy moss can quickly become de-oxygenated, as the invasive surface weed acts as a barrier between the water and air. 

Spawning time

In the summer time your koi may be “jumping” for either flies on the water’s surface or through the vigour’s of spawning activity.  The summer provides a short window in which koi can breed and their fry can grow to a size at which they will survive a winter.

Spawning activity in a koi pond can be quite a physical process, where the attentions of the males on a ripe female forcing her to release her eggs can result in these koi becoming exhausted and damaged, especially when there’s an absence of lush aquatic vegetation.  The chasing and bashing associated with spawning activity can go on for many hours, and will result in many thousands of tiny translucent and adhesive eggs being deposited on submerged surfaces.


Summer represents the final scene where the last battle between good and evil takes place.  Will it be victory to green water and blanket weed or will a balanced pond with healthy desirable plant growth outcompete the slimy green curse?

It is a very rare pond that has no trace at all of either green water or blanketweed.  In fact, the way these two undesirable algae interact means that a pond will either show signs of one or the other.  When green water is in abundance, it shades out and utilises the light and nutrients that blanket weed would thrive on.   Yet if through means of an algicide or ultraviolet steriliser, the green water is eradicated, then blanket weed is likely to thrive in such conditions.  If blanketweed does gain the upper hand (which is often the case in an unplanted koi pond) then its growth is prolific.  

Top up?

Evaporation may be quite significant in extreme summer conditions, often exposing an unsightly ridge of pond liner. 

If you are using a bog garden as a type of vegetable filter, adjacent to the pond, then they are particularly prone to drying out, as the submerged soil can act as a wick and draw moisture up and out of boggy areas. 

On a fortnightly basis, besides just topping up for water lost through evaporation, it is valuable to the health of fish, especially now they are actively feeding that some of the pond water is actively removed and replaced with fresh.  When water evaporates it is only pure water that is lost, leaving behind an accumulation of other soluble compounds.  If the water were to be simply topped up each week, then these compounds are likely to become so concentrated as to affect water colour and fish health.  As long as a maximum of 30% is changed at any one time, tap water and the use of a tap water conditioner to neutralise the toxic chlorine is quite safe.


So strange (yet predictable) is much of human behaviour, that summer is the time for 2 weeks away in the sun; leaving the pond on its own, when it is at its best (and perhaps most vulnerable).

Find a reliable neighbour with a list of things to check. 

Pump pre-filter, fountain head, water level and blanket weed growth should all be on a list of things to check.  To prevent a well-meaning neighbour from over feeding your koi, put a sealed portion for each day to one side for them to feed.

In summary, the summertime is the high point of koi keeping, when your koi and any plants should reward you for all of your efforts in the run up to summer.  It is also a time to be vigilant, as water quality and algae problems can soon get out of control!

Kill blanketweed and string algae.