Different types of fish have different behaviours.
Livebearers: all-rounders which dwell at any depth and don't need to be kept in shoals of their own kind - guppy, swordtail, platy and molly.
Surface feeders: dwell just below the surface. They have straight backs and upturned mouths - zebra danio, Siamese fighting fish and hatchet fish.
Mid-water feeders: have small mouths pointing forward - including angelfish, harlequin fish and pearl gouramis, glass catfish and goldfish.
Bottom dwellers: flat bodies and downward facing mouths - e.g. bronze catfish, pleco and clown loach.
The number of fish you can keep in an aquarium depends not only on the water quality but also the behavioural needs of the fish. Keeping too many fish in the tank means they'll have to compete for food and they may become stressed as a result.
- Substrate – make sure it's suited to your fish and wash it well before use.
- Plants – provide shade shelter. It's better to provide real plants than plastic as they release oxygen and nutrients into the water.
- Shelter – Branches and sturdy rocks provide shade and shelter - important for your fish to feel secure. Hides and caves are essential for some catfishes.
Remember that you need to provide furnishings and enrichment to provide for the specific species behaviour. For example, kuhli loaches and many catfishes will need caves and hides if they are to feel secure.