When it comes to keeping Apistogramma, the general rule of thumb is to only keep one species per tank. The reasoning behind this is that these fish naturally form small schools in the wild and do not usually mix with other species. Keeping two or more different Apistogramma species together can cause aggression between them as they compete for food and territory. It's also important to bear in mind that many types of Apistogramma are territorial and will establish their own areas within the aquarium. Therefore, if too many specimens are kept in a single tank, there may be no room left for each individual fish to establish its own territory which could lead to stress-induced health problems down the line. Ideally, you should keep at least 6 individuals so that they have space enough to move around freely without crowding each other out but beyond this number it might be best not to go any higher as overcrowding could become an issue even amongst just one type of Apistogramma species.