Filtration helps keep the water clean to a certain extent, but you still need to keep an eye on the water perimeters of your fish tank.
Continuous water testing, monitoring and correct water changes, should help take care of your tropical fish water.
This helps (depending on the size and mix of the fish community and the species of your fish) sustain your tropical fish in a healthy environment.
This is of coarse must be within the correct water parameters (i.e.correct temperatures, the condition and make up of the water, for the particular species of aquatic animals and plants you have in your aquarium.)
Regularly check for a rise in nitrogen compound: Ammonia - Nitrates - Nitrites
An increase in parts per million (ppm) of ammonia to water is a result of nitrification from bacteria breaking down aquarium waste