A healthy fish is normally lively (except in cold weather) and has an erect dorsal fin (the one on the back). If any are sluggish and have drooping dorsals they are off colour. There could be many reasons, but the commonest are insufficient oxygen and foul water. The water can easily be oxygenated and freshened up by the splash of a waterfall or fountain. Tracing the cause of water toxicity may be more difficult. More often than not it is produced by the decay of vegetation – particularly fallen– in the pool. All are to the extent that when they rot in water small amounts of toxic gas are released as the by-products of decay. Whether or not it is a serious danger depends on the amount of decaying material in relation to the size of the pool. Large leaves like sycamore and chestnut are bad; willow and poplar leaves are worse; the and , as well as the leaves, of laburnum are really poisonous. Any such accumulation must obviously be removed and in future seasons prevented as far as possible.
Fouling can also be the result of spray-drift from insecticides used near the pool, from weedkiller washed off the adjoining lawn, or sodium chlorate used on a nearby gravel drive. Such pollution of the water, if not immediately deadly to the fish, will certainly lower their resistance and make them vulnerable to infection by organisms that are present in the water in the same way that germs are always present in the air around us.
Nothing can be done to eliminate these organisms by treating the water. Fish are unaffected by them unless their resistance is undermined by overcrowding or unhygienic conditions; an unsuitable or inadequate diet; sudden changes in temperature; insufficient oxygen; and exhaustion after spawning, as well as pollution of the water.
The commonest of these organisms are the spores of Fungus Infection (saprolegnia) which are present in all water. They infect fish which are out of condition and also develop rapidly in wounds caused by cats or birds, producing whitish or grey threads on the skin or fins or gills, sometimes abundant enough to resemble tufts of cotton. The tufts may be stained green if the water is soupy, but the disease is in no way connected with the thread-like algae called blanketweed.
Treatment for Fungus Infection is to place the fish in a shallow dish to which salt has been added at the rate of two heaped teaspoonfuls per gallon of water (sea salt, if obtainable, is much better than table salt). Keep the fish in thefor three days, changing the water daily and (unless the fish shows sign of distress) increasing the salt content by one teaspoonful each day. After three days hold the fish in a damp cloth and, using a soft brush dipped in weak iodine (solution 1 in 10), brush off the fungus, taking care to avoid touching areas of healthy skin. Return the fish to the pool; the treatment can be repeated after a week if the infection persists. The disease is not infectious from fish to fish. Changing the pool water may clear any suspected pollution, but will not get rid of saprolegnia spores because they will be present in the new water too. Infection is most common in spring before fish have recovered from the effect of their winter fast.
The salt bath treatment described above (without the iodine-brushing part) is used for some other ailments. Details of the other treatments referred to there are as follows:
Epsom Salt Bath. Shallow dish to which Epsom Salts have been added at the rate of three tablespoons per gallon of water.
Running Water Treatment. A shallow dish standing in the sink with water from the tap running into it at the lowest pressure that will give a steady flow. A few drops of iodine may be added to the water in the dish and the fish placed in the solution for an hour before the tap is turned on. Disinfecting the Pool. For every 100 gallons of water in the pool to be treated, mix 30 grains (2 grams) of potassium permanganate (permanganate of potash) with 1 pint or so of water. When the crystals are thoroughly dissolved, pour the solution into the pool. Never put the crystals directly into the pool. Repeat the treatment after ten days. Keep any fountain or waterfall going during treatment to assist aeration. This treatment will not harm plants or fish, which can remain in the pool. The water will be deeply coloured but this will fade rapidly and disappear completely in a few days.
The value of disinfecting the pool in this way is debatable. There is certainly no chance that it will eradicate such organisms as the spores of saproleg-nia. Nevertheless I believe it to have an inhibiting, if not completely destructive, effect on the free-swimming juvenile stages of some parasites. What it boils down to, I suppose, is that it is not going to do any harm and it may do some good, and it is more satisfying to the pool owner than doing nothing.
It has been said that a sick fish is a dead fish. While this is an exaggeration, it is true that some of the diseases (as opposed to the parasites, which are relatively easy to deal with) are unlikely to yield to treatment if they are not caught at an early stage. If a fish is clearly beyond recovery it can most mercifully be destroyed by throwing it hard against a concrete path or similar surface.