Bottom water fish are those that prefer living at the lowest level of the aquarium. It's not that they can't swim into the upper regions, they will when spurred by a fish that pesters them, or just for the sake of a quick dash around the tank. But for the most part, they live on the bottom, which is where most of their food comes from.
Tropical fish that prefer the bottom of the tank, usually eat algae that grows there, as well as leftover food that falls on the substrate or the broad leaves of some plants. In a way, the bottom water fish are the housekeepers of your aquarium, cleaning up scraps and preventing the build up of algae. But this is not always a sufficient diet, and they need to be given food that comes in a form or shape specifically designed to reach, and appeal to the bottom feeder.
This is usually a wafer shape, dense enough that it sinks past the top and middle feeders, to rest on the bottom and soften. The bottom feeders can then browse at their leisure, returning later to clean up the remains. In that respect, they are not like top and middle feeders, where food is given a pinch at a time, and feeding should stop as soon as they lose interest. Those tropical fish that hang around the bottom tend to be "grazers", and not the gulpers that you'll find dashing for the surface when they see you coming.
Some of the better-known bottom water fish are the loach, and catfish. There are also algae eaters, Botia, Corys, Knifefish and the more unusual specimens like Goby and Needle Nose.
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