Essential Koi Pond Goldfish Health Checks: How to keep pond fish in top condition.

Many water gardeners or pondkeepers will inevitably dabble in fish at some time or other. Lured by the attraction of a dash of living, breathing and moving colour or tempted by the choice of a myriad of red, golden, silver and blue pondfish teeming with vitality in well presented tanks in the local aquatic store.

How can we ensure that the vigour and colourful vitality of the fish which we long to take home will continue once we’ve bought them and introduced them into our own pond?

By making water quality a priority for our ponds, once introduced the fish will more or less look after themselves. They will not be stressed or easily succumb to disease and as long as you keep an eye on a number of key aspects you will be rewarded by happy, healthy fish. Prevention is better (and cheaper!) than cure.

The Fish.

Choose wisely, and only buy fish in small numbers, building up a collection of a range of complementary shapes and sizes. This will allow your pond to adjust to the new inhabitants and the various effects they will have on the ecosystem within the pond.

Water Quality

Natural is best. A well-planted, mature and balanced pond or water feature will provide an excellent nearly-natural environment for fish. A pond stocked with a variety of marginal, surface and submerged plants will act as natural water purifiers, conditioning the water for other aquatic life, including fish.

If stocking rates are minimal and at a natural self-sustainable level then the fish and plants will form a mutually beneficial balance, causing your pond fish to flourish.

\ Most ponds, however, end up being stocked with more fish than can be kept healthily on a self-sustainable basis. This is particularly true in koi ponds where potted plants will be uprooted by the inquisitive nature of the koi. In such cases, fish health will suffer without extra aeration, circulation and filtration as their waste production will exceed that which a planted and balanced pond can naturally breakdown.

Filtration is the secret behind the fabulous displays in aquatic stores where hundreds of fish apparently thrive in such high stocking levels. What we don’t see are the belt ‘n’ braces filtration systems behind the scenes.

We need to do the same in our ponds and by installing a filter we can keep more fish in a healthy, stress-free and growing condition than would be possible on a natural basis.

Why does a pond need a filter?

Most ponds need a biological filter that will breakdown the toxic waste (ammonia) produced by fish into nitrites (which are still toxic) and then into less toxic nitrate. The friendly bacteria that colonise the filter detoxify the ammonia making the water safe for fish. The filter should never be switched off, as it is full of living bacteria that are kept alive by a constant flow of oxygenated water. Filters can also be fitted with a UV clarifier which irradiates pond water, guaranteeing crystal clear water. A UV clarifier produces extra solid waste so always fit a slightly oversized filter if also using a UVc.

Pond Space. Bigger is better.

A pondkeepers priority must be with the water quality, just as successful gardeners pay a lot of attention to the soil. If the medium in which the fish are growing is in top condition then success will follow. If the water quality is well understood and managed then we do not have to be experts in fish or even disease as the fish’s health will follow automatically and are not likely to experience disease.

Fish prefer stable water conditions and if any changes do occur within the pond, then they must be gradual. It is far easier to keep fish in a larger pond as it provides a larger volume for diluting any problems (the solution to pollution is dilution) and a larger surface area also allows effective gas exchange. A larger pond also produces larger fish, as they grow to the size of the pond.


It is essential that pondfish receive a well balanced diet that will satisfy their energy and growth requirements in the warmer months and allow them extra energy to store in order to overwinter satisfactorily. A quality food is also important in that the formulation is stable with no tendency to cloud the pond water.

Many foods now contain additives to enhance health and colour and tests have shown that by feeding certain supplements, the red pigmentation in goldfish and koi can be enhanced to a deeper blood-red colour. Why not have a go with your own fish and see what happens to their colour?

Colour Enhancers – Did you know?

Feeding colour-enhancing foods can enhance the colour of your pondfish. These are natural foodstuffs that contain high levels of pigments, causing fish to take on that colour. Colour enhancing additives that are found in high quality pond foods include paprika, krill, marigold meal and algae!

Let your fish tell you what’s happening in your pond.

In a way, the fish act as a ‘barometer’ for the health of a pond. Watching and enjoying the behaviour of your fish is key to maintaining a healthy pond full of fish and will allow you to predict any problems before they escalate. Fish watching is a key responsibility for any fishkeeper as a change in behaviour such as not feeding, sulking on the bottom etc will indicate that the water quality is not ‘sweet’. At the first sign of a change in behaviour, stop feeding and test for ammonia or nitrite to see if the filter is coping with the waste that the fish are producing. The fish are a barometer for the health of a pond.


Install a biofilter

Watch your fish regularly for any telltale changes in behaviour.

Feed a good quality food that contains colour enhancers

Choose fish wisely. Only buy fish that look healthy in the shop tank.

Ask for advice at your aquatic shop for any worries or queries you may have over your fish’s health in general.


Ever switch off the biofilter. It is the heart and lungs of a pond and switching it off will cause fish to suffer through a drop in water quality.

Stock too many fish in the first go at stocking. Introduce fish to the pond gradually, bearing in mind that the filter takes time to become established.

Feed the fish if they show signs of a change in behaviour. Test the water for ammonia/nitrite levels to see if either is present. If they are, the fish will be stressed. Carry out a partial water change and monitor the ammonia/nitrite levels over the next few days.

Kill blanketweed and string algae.