Anyone who keeps fish is aware of some basic ideas about the niche of the fish they keep: whether the fish lives in fresh, salt, or brackish water, its optimum temperature. More advanced fishkeepers are also aware that their animals have an optimum pH and water hardness – for example the rift valley cichlids live in alkaline water. The diet of the fish also corresponds to its trophic level.
- Nutrient cycles
Fishkeepers also become aware of the effects of inadequate filtration and the necessity of maintaining proper recycling of nitrogenous wastes. Nitrate, nitrite, and ammonia levels are all something aquarists learn to test for. Fishkeepers use biological filtration, i.e., a reservoir of bacteria in a filter or in the gravel bed, to avoid high ammonia levels.
- Behavioral ecology
Some fish display territoriality and some are fine to keep with other fish. Some display shoaling behavior. Some fish will dig in the substrate. And of course there are dither fish.
All in all, keeping an aquarium predisposes a person to learning some basic ecological concepts. If you were a careful enough observer and wanted to try some practical experiments, you might be able to observe something like exponential growth, say if you accidentally introduced snails with some plants.