Freshwater

Freshwater aquarium equipment works as one system, so the parts have to match. Filter and heater output are chosen by volume in litres, and the light by the tank's length and how demanding your plants are. If the equipment is too weak for the tank, careful maintenance will not make up for it: the water clouds and the temperature drifts.

The second decision is whether the tank is mainly for plants or for fish. A densely planted aquarium needs a nutrient-rich substrate, regular fertiliser and often added CO2; a fish tank does well with gravel, a good filter and a few undemanding plants.

The most common mistake is stocking too quickly. A new filter has not yet grown the bacteria that break down fish waste, so add fish gradually and check ammonia and nitrite with tests. After that, a weekly partial water change keeps the water clean.