What size is an "aqua one 620t"? I've never heard of this. But the minimum size I'd reccomend for long term keeping africans is a 55g (4ft long). Also, you should crowd it to reduce aggression, but make sure you have enough filtration to handle it. In a 55 you could put 10-15 African mbuna cichlids, depending on what type you get. Check out Yellow Labs, Acei Cichlids, Powder Blue Cichlids, Cobalt Blue Cichlids, Maingano Cichlids, and Afra Cichlids, they are all relatively laid back for mbuna (which is still aggressive). Also Demasoni are beautiful, stay small, but are very aggressive to each other, so with them you would need a minimum of around 10. Generally it is best to keep africans in groups atleast 5, and a minimum of 2-3 female per male, more females the more aggressive the species. I think most people pick 2-3 species with nice contrasting colors and and get atleast 5 of each. One other thing that can work is to get only one per species, since they don't tend to be as aggressive to fish that don't look like them, but if you do this it is essential to only mix species with similar temperaments, who do not look too much alike, and who are from different Genus (with the exception of psuedotropheus, which is a catch all right now for unclassified species). I realize I have written this whole thing on Mbuna, but you didn't even mention what type of Africans you were wanting to keep. Shell dwellers, peacocks, etc all have different requirements. The diversity and complexity of Africans kept me from keeping Africans for a long time just because it all seemed so confusing. Just do a lot of research!