Apple snails get only 2-3 inches long and you should make sure you have some algae on the glass, decorations, plants, etc. before you introduce one to your tank. They are constantly looking for something to eat and alot of times you'll see them at the top of the aquarium with their siphon sticking out to breath in air. They will eat algae, uneaten fish food, frozen foods, live foods and just about anything thats edible including plants. They are either male or female, so as long as you have one snail, two males, or two females, they can't multiply like other snails.
Also, depending on how much algae and detritus is in the tank, they can produce alot of waste. I had one that I kept in my ten
gal., but once it cleaned out the whole tank, I decided to move it to my 55
gal. That was a big mistake. My blood parrot would never let it get to the top of the tank as he always attacked it. After about two days, I saw an empty snail shell on the bottom of my tank
I have two clown loaches in that are about 5 in. long each. Can clown loaches eat apple snails? My apple snail was about 2 incehs in diameter.