Fish disease treatments are often the main threat

Fish disease treatments are often the main threat

Often the biggest threat to koi health are fish disease treatments! How do most disease treatments work? Should you treat the pond or the fish?  Whatever you do take precautions.

Pond “medications”?

There are several considerations when using a pond ‘medication’ but undoubtedly the most basic is to understand their true nature. Pond treatments are often referred to as ‘fish medications’, a misleading term. With few exceptions the most commonly used treatments are not medications in the accepted meaning of the word – that is, compounds specifically formulated to be used in the treatment of disease.

Most chemicals used for fish treatment are toxic, taken straight from the chemist’s laboratory and normally used for other purposes. They work as ‘fish medications’ simply because they are potentially toxic to all forms of life, but tend to kill the smaller organisms, such as fish parasites, before the larger ones, such as koi!

Safety margin

If we get the dosage right then we can kill micro-organisms such as parasites and bacteria before doing any serious damage to larger animals, such as koi. However, I should stress that often the borderline between safety and toxicity is very small and every year thousands of fish die as a direct consequence of chemical overdosing. Because of the way that these chemicals work it is virtually impossible to reverse the effects of overdosing, the exception being salt, which works in a different way.

If my comments above have changed your perception of chemical treatments then the most important lesson has already been learned. When administering chemical treatments to the pond it is important to realise beforehand the potentially devastating effects if you get it wrong. Respect and caution are the golden rules to observe when handling the potentially dangerous chemicals that many ‘fish medications’ are.

Treat the pond or fish

The first decision is whether to treat the pond or the individual sick fish. Unfortunately, there are no hard and fast rules, it depends on the circumstances. In general, treatment of an individual fish is to be preferred. This is particularly true where bacterial, fungal or gill problems are suspected. But where there is a generalized parasite problem, pond treatment is preferable, in order to treat the various stages of parasite development and reduce the overall parasite population.

This obviously presumes that any remedial action has already been taken to isolate and correct any environmental or other stressors responsible for the underlying problem. Using treatments without correction of underlying problems is simply a waste of time, usually resulting in yet more stress for the fish and further health problems.

If just one or two fish are affected, then it is probably better to treat just those fish in a tank or a bath, returning them to the pond after treatment. If a succession of treatments is required, then the affected fish can either be permanently isolated in a separate tank or returned to the main pond between treatments. Individual treatment has several advantages in that any treatments can be better controlled and the fish more easily removed if it reacts badly. It is also cheaper in terms of chemical costs and it avoids the risk of upsetting the delicate eco-balance established in the pond and filter.

The use of a short-term bath also enables higher, and therefore often more effective doses to be given. If fish are to be removed for longer-term treatment, e.g.  where there are bacterial problems, then a suitable quarantine tank of adequate size is an essential requirement. Trying to treat fish in a small, badly designed tank is pointless, as any beneficial effects will be quickly negated by the stress caused by such poor environmental conditions.


When handling any chemicals, always wear gloves. As already mentioned, some chemicals are poisonous and may irritate the skin. Mix the required amount in a bucket or similar, to ensure that it is fully dissolved. This is particularly important with tank treatments as uneven mixing could result in areas of high chemical concentration and therefore higher toxicity.

Always aerate tanks while treatments are in progress; cover the treatment tank to prevent fish leaping out and NEVER, EVER leave the tank during the initial stages of treatment. Bear in mind that you are dealing with a sick fish that may not be able to stand up to the treatment or alternatively the diagnosis may be wrong and the treatment may exacerbate the problem.

It is important that treated fish are watched immediately following treatment. Have a tank of clean, aerated water to hand in case a fish reacts badly. On a practical note, a sock-net is invaluable for transferring fish between pond and tank. These nets remove the risk of dropped fish, making handling less stressful for both fish and handler. When the treatment time is complete, ideally the fish should be returned to a tank of clean water, rather than being put straight into the pond -this enables the keeper to monitor its progress and check that the fish is recovering well.

Isolate the filter

Pond treatments, if they are to be administered with minimal risk, need careful thought and preparation. The first consideration is the filter system. I have heard it said that chemicals attack only ‘bad’ bugs and have no effect on ‘friendly’ bugs. A moment’s thought suggests that this is highly improbable. The general undiscriminating nature of these chemicals means that both good and bad bugs are likely to be killed so, ideally, the filter should be isolated while the pond is being treated. How easy this is in practice will obviously vary from system to system. In the conventional gravity-fed filter it is relatively easy to block off water feeds and returns and set up a small pump to continue to circulate the water within the filter. In this way a filter can be isolated from the pond, for days if necessary. Koi-keepers using other types of filter system may not find it quite so easy to both isolate and keep a filter running.

I am thinking of box-type filters in particular, where it is impossible to continue to circulate water in isolation from the pond. Even with this type of system, however, it is still possible to isolate the filter for some time without having an adverse effect on filter activity. Provided that the media are kept submerged by either plugging the water return or removing the media to a large tank filled with pond water, the filter bacteria will remain viable for a considerable time. A couple of air-stones under the media will give some water circulation as well as ensuring a continuing oxygen supply.

How long a filter needs to be isolated depends on the type of treatment used and how quickly it breaks down. I would suggest a period of several hours, say, overnight. Obviously, if the filter is isolated, and therefore water returns switched off, supplementary aeration is vital. Any additional aeration needs to be quite heavy, as some chemical treatments, particularly those containing formalin, remove oxygen from the water. A Hi-Blow  air pump is ideal for this purpose.

A good clean

Another simple precaution before dispensing a treatment is to give the pond a good clean, getting muck and waste off the bottom. I can sense a few puzzled looks at this piece of advice but a little thought shows that it makes good sense. Many chemicals used for pond treatment will react with organic matter such as fish waste and detritus. Giving the pond a clean before treatment ensures that more of the chemical is available for therapeutic use rather than simply being wasted in unwanted chemical reactions. This is particularly important with potassium permanganate, which can be rendered useless in a dirty pond. A clean pond also reduces the risk of water quality problems while treatment is in progress, particularly if the filter has been isolated.