Tajaba--Well, you could fit plenty of them!
Apistos alone, with the current filtration, and all other species that are currently in there removed, I would guess that you could comfortably have at least 150 Apistos in this tank. IF your goal is to breed, that I probably wouldn't put any more than 15-20 pairs in there, but that's an educated guess. You could make this Apisto Heaven though!
Now I see
Thanks for detailing the O2 issue!