In a fish tank, its size, the number of the inhabitants, the presence or absence, of snails and vegetables, and the source and nature of the water supply, are important factors. Several gold fish and carp would in a small tank soon exhaust the supply of oxygen, while their products would still further impoverish the water. If a white powder covers the bodies of the fish it is of fungoid growth, but the cause has been much debated. Still, it is generally agreed that nitrogen is necessary for the growth of every kind of fungus, and therefore it is reasonable to say that nitrogenous matter in solution must be in excess. This may be the result of the decomposition of animal matter, and the best way to correct it is by adding oxygen to the water mechanically by causing a circulation of water, either by a fountain playing and thus entraining air among the falling drops, or by allowing a stream of water to pass through the tank. A cruder method would be to suspend a leak-ing vessel above the tank, thus allowing water to fall, taking air with the drops. A natural method would be to add growing water-weeds, and thus allow them to give oxygen to the water. In some cases lime in the water has been held to be a cause of the powdery appearance, but lime is not a necessity to fungi.

The affected fish may be quickly cured by placing them in a vessel into which water is constantly dripping from a tap.