Should add that - as you already know - zebras are really tough cookies, so if you can't put more zebras in your tank, makes you wonder what you COULD put in there. Depending on other factors like filtration, plants and water changes, you could add 2-4 zebras and put in a couple of dwarf corys, or maybe 4 dwarf corys and 2 more zebras? Personally, I'd put even more in there of each breed, but heck, I don't want to get my wrists slapped around here for giving dodgy advice! LOL!