The dwarf cories that stay small enough for your tank would be hasbrosus and pygmaeus, those are the tiny guys that may be harder to find, and there's julii or even panda that would be OK and are easier to find. Price varies, my petco charges 4.99 I believe for the julii, my mom & pop lfs charges 6.99...I see them online for much less so there's no telling what it will be in your area. If you see the cheaper albino or green ones and think of getting them instead..remember these get way to big for your tank. I would go with 4 dwarf cories for your size tank. Substrate is important with these guys. No sharp gravel or it will destroy their barbels and they will starve to death.