I have 3 (constantly) breeding pairs of cutteri in a 90, along with a handful of small SA cichlids and 6 P. typus catfish to eat the thousands of fry. I'd really recommend those rather than convicts in a 20 high. They claim a small territory and are much less aggressive in general. You wouldn't be able to keep 3 pairs of convicts in a 90, much less other fish too, w/out mayhem and death.