You have lots of options when it comes to a clean up crew, and I'm sure there is a thread on here that discusses it in depth, but crabs and snails are usually your main players. Personally I prefer snails, crabs can be great but even when I would provide tons of empty shells in a tank, the crabs end up killing the snails.
I would only add a few to start though, there are tons of packages out there but usually thats way more than you need. I would grab a nassarious snail, a trocus snail and as algae becomes more prominent in your tank, 1 mexican turbo wouldn't hurt (they eat tons of algae so you only need one in your 20 gallon).
I would only add a few to start though, there are tons of packages out there but usually thats way more than you need. I would grab a nassarious snail, a trocus snail and as algae becomes more prominent in your tank, 1 mexican turbo wouldn't hurt (they eat tons of algae so you only need one in your 20 gallon).







