A new aquarium needs four to six weeks for filter bacteria to develop before fish are added, and fish are then introduced a few at a time. Most early losses are caused by ammonia and nitrite, not disease.
Weekly partial water changes, light feeding and rinsing the filter only in tank water keep an established tank stable.
A tropical aquarium is a small, closed ecosystem. Most fish that die in new tanks are killed not by disease but by water that looks clean and is chemically poisonous. Understanding how a tank processes waste, and being patient at the start, is most of what a beginner needs.
Fish excrete ammonia, and uneaten food and dead leaves release more. Ammonia is highly toxic. In an established tank, bacteria living in the filter convert it first to nitrite, which is also toxic, and then to nitrate, which is far less harmful and is removed by water changes and taken up by plants.
A new tank has none of these bacteria. Building up the colony, known as cycling, takes around four to six weeks. The humane method is to do it before any fish are added, by running the filter and heater and adding a source of ammonia, and to test the water until ammonia and nitrite both read zero. Fish are never bought on the same day as the tank.
The tank is placed away from direct sunlight, radiators and doors. Electrical equipment is plugged in with a loop in each cable below the level of the socket, so that water cannot run down it, and hands go into the water only with the heater and filter switched off.
Fish are added a few at a time over several weeks, so that the bacteria can keep pace. Hardy, peaceful species for a first tank include platies, guppies, zebra danios, harlequin rasboras, cherry barbs, small tetras and corydoras catfish. Shoaling fish are kept in groups of at least six of their own kind.
Before buying, the adult size, temperament and water requirements of each species are checked. Some fish sold small grow very large, some nip the fins of others, and fish from soft, acid water do poorly in hard tap water. Goldfish are coldwater fish and do not belong in a tropical tank. A good aquatic shop asks about the tank and will test a sample of its water.
Overfeeding is the commonest mistake. Fish are given only what they finish within a couple of minutes, once or twice a day, and healthy adult fish come to no harm if they miss a day. A varied diet of flake or pellets with occasional frozen or live foods suits most community species.
The whole tank is never emptied and scrubbed.
Signs of trouble are clamped fins, gasping at the surface, rubbing against objects, white spots, frayed fins, bloating and loss of colour or appetite. The first step is always to test the water, since most disease follows poor water quality or stress. Raised ammonia or nitrite is treated with immediate partial water changes.
White spot, a parasite that looks like grains of salt on the body and fins, is the most common infection and responds to proprietary treatments used as directed. New fish are best kept in a separate quarantine tank for two to four weeks before joining the others, and nets and equipment are not shared between tanks. Dead fish are removed at once.
For absences of a week or less, healthy adult fish can go unfed; for longer, a neighbour is left pre-measured portions, since a well-meaning helper usually overfeeds. In a power cut the tank is wrapped in blankets to hold the heat and the fish are not fed until the filter is running again.
Unwanted fish, snails and aquarium plants must never be put into ponds, rivers or drains. Releasing non-native species is illegal in the United Kingdom and has caused serious damage to waterways. Aquatic shops and fishkeeping societies will often take fish that have outgrown a tank.