90 liters = almost 24 gallons. I have a planted 20 high with guppies, neocaridina shrimp, and snails and it is a very rewarding tank to keep. I also have a planted 40 breeder with blue dream neos, yellow tiger endlers, an otocinclus cat, and snails. I'm very happy with both tanks. Theres always something going on. If you want Africans, neolamprologis multifaciatus are shell dwellers that would work well and they colony breed. For a Central American cichlid, I might try rainbow cichlids. Theres a lot of directions you can go with a small tank and make it cool! If you want easy I'd highly recommend neocaridina shrimp, Malaysian trumpet snails, and plants if you have the right water. They can be very low maintenance.