There are of course many species in the Apistogramma
genus, and while most are around the same size, their numbers do vary with the species (pair, trio, group). I would need to know the tank length [100 liters, which is around 26 gallons] to pin this down further. A. cacatuoides
is (so far) the only Apisto species in our profiles, here's the direct link: http://www.tropicalfishkeeping.com/c...uoides-189393/
and you willnote therein it mentions that this is a harem species so one male with 2 (minimum) or 3 females is best. This is for a reason, mainly the male's temperament. Tank size is obviously critical here.
There are other apistos that are fine in a pair. And moving to other genera, the beautiful gems in Dicrossus
are well suited to smallish tanks like a 20g and up. The two most commonly seen species of Dicrossus
are in our profiles: http://www.tropicalfishkeeping.com/c...ntosus-189241/ http://www.tropicalfishkeeping.com/c...ulatus-189257/
Dither fish are advisable or all of these cichlids may become so frightened they refuse to eat and waste away. Substrate fish are not always ideal with these cichlids especially in small tanks; the cichlids will regularly spawn, and they find substrate fish bothersome and the females when guarding eggs/fry can be very nasty to corys. And, if you want fry to survive, never have substrate fish; substrate fish like corys, pleco etc. are nocturnal and will easily devour eggs or fry at night even in large tanks.
Upper fish are fine, so long as they are themselves peaceful. Pencilfish usually do well in this role, and there are some tetra than do as well. Hatchetfish too are ideal, esp those in Carnegiella
which are smaller and quieter. All of these are in our profiles under "characid species."