This article is by Syd Mitchell - K.O.I.'s Water Quality Instructor - often nominated for Sainthood by his students...  Syd discusses chemicals used to treat a wide variety of problems...

Many koi keepers reach for a bottle of their particular favourite medication at the first sign that their koi are behaving in any unusual way but is this course of action the best approach to adopt? I would say not.

OATA estimates that 90% of fish deaths are primarily caused by incorrect water parameters. This doesn’t mean that in every case they will have been poisoned by bad water. Some will have died as a direct result of poor water quality but the majority will have been weakened by it so that they fall victim to something they might otherwise have been able to resist.   If koi look or act as if they are unwell, it’s always advisable to firstly ensure that all water parameters, especially the oxygen level, are acceptable and to quickly correct any parameter that isn’t.  This may well prove to be all that is necessary.

If water parameters are satisfactory and the fish still look unwell, the next action to take is a proper diagnosis before treating even if this means calling for help from a professional or someone with greater experience.

Adding a treatment to the pond in the hope that it will be the right one isn’t advisable.  Formalin and malachite green (FMG) is effective against a range of koi parasites but it isn’t a “cure all” treatment that is effective for all ailments. If this was the case there would be no need for koi dealers to stock anything else. As with other medications it has its uses but it also has side effects.  There are sufficient indications that malachite green is carcinogenic for it to have been banned as a medication for food fish because it breaks down into a compound called leucomalachite green which persists in such tissues as their liver, kidneys and muscles long after they have been treated.  There are no studies that I am aware of into whether tumours in koi can be caused by leucomalachite but it seems sensible not to use the treatment more often than is really necessary.

Another effect of FMG is that it lowers the oxygen level; each 5 mg/L of formalin removes 1 mg/L of dissolved oxygen from the water. A fish gasping at the surface may be exhibiting a symptom that it has been infected by a parasite or it may simply be a sign that the oxygen level is low.  Treating the pond with FMG in such a situation and lowering the oxygen level even further may be the last straw resulting in the rapid deterioration of the fish’s condition or possibly even its death.

Potassium permanganate is another chemical that is frequently used at the first sign of trouble but it’s aggressive and shouldn’t be used without care. Last year, at a koi show, a mobile phone was thrust into my hand so that I could give advice to someone who had treated his pond with potassium permanganate and whose koi immediately began dying.  The story is in the health pages of this site and, in essence, it was about a man who followed some poor advice about dosing with potassium permanganate at the first sign of trouble but without the warning that it’s a very aggressive chemical and shouldn’t be used unnecessarily or without due care. It’s a good example of the dangers of over using a chemical at the first sign of trouble.

By: Syd Mitchell