W
werd
Guest
lots of reasons. first, for the biological , mechanical and chemical filtration of the water. im sure you know nothing of the nitrogen cycle. essential , a small container like a bowl will never fully 'cycle' therefore it doesn't require a filter. a large container of 3 gallons or more needs to go through a cycle so that the natural bacteria in the tank can take care of the things like nitrogen and ammonia which are harmful to the fish. a filter has chemical filtration which depletes ammonia, mechanical filtration which takes care of larger waste and bacterial filtration to make the water safe and livable for the fish. the second reason is aeration of the water. you need oxygen for the fish to survive. certain fish like goldfish and beta have what is called a labyrinth organ which helps them breath air from the water's surface. most other fish take oxygen from the water through their gills. this is why oxygen in the water is so important. i hope this answers your question =)