Bottom eating fish are fish that would be catfish, otocinclus, any fish that scavenges off the bottom. Remember, fish in an aquarium are going to eat everywhere. They're going to go to the top of the tank, the bottom of the tank, they're going to nibble on the decoration. They are even going to try and eat the algae on the glass. In an aquarium, you're going to want a fish that predominantly eat on the bottom. Any fish with the morphology of a down-turned mouth, usually is a fish that would scavenge off the bottom. These fish are critical in an aquarium, because they are going to eat a lot of the leftover food. Now, that doesn't mean that you can overfeed, but you should have fish that scavenge on the bottom. Any of the corridors, catfish, pleckos, otocinclus; those are auto-cleaning fish, but they also scavenge waste and uneaten food off the bottom.
Another beneficial thing to have bottom-eating fish is, when they're going about their business, they are kicking up a lot of the waste that settles on the bottom gravel, and allowing the water column to pick it up, where the filtration can eventually filter it out. Having fish with little feelers on their mouth is great, because they're going to go on the gravel and they're going to free up a lot of the waste.
Food, when you feed an aquarium, that food is really only good for an hour or so. After that, it turns to mush. So, it's not like these fish are going to be eating the food all day and all night, but they are helping to probe the bottom of the tank. You want to keep it visually looking great; you don't want to see all the detritus and waste that settles on the bottom. They're beneficial to the aquarium. Any of the small catfish, pleckos, algae eaters, otocinclus; those are the fish you want to go with. Again, they do contribute to the total fish volume in your aquarium, so if you like those mid-water, beautiful, showy fish, only go with a couple of bottom fish, and make sure they're not catfish that get too big. A lot of catfish get really, really big, so you want to make sure that you don't go with anything with a really big mouth; it's going to eat your other tankmates.