In a 20L, personally I'd be looking to the cryptohero genus from Cetral America. I have raised and spawned a few species in 20L's without a problem. As for the more colorful species, I'd look at Honduran Red Points, Myrnae, Sajica, Septemfasciatus or Nanoluteus. Herotilapia Multispinosa "rainbow cichlids" could work as well. I used to keep apisto's in 20L's but have since moved them to 30 breeder and 50L (36x18 and 48x18) they have exibited much better coloration, and more intresting behaivoir plus more spawning. Just cause some cichlids are small doesnt mean they can live in small quarters. Some apisto's can work well, others need more territory, especially if they are harem spawners. The crypto's I suggested, get a bit larger (4" average, 6" exceptional) but even in large tanks, stake out very small territory. Check aquabid, most of those are usually readily available. I left out convicts as they're not quite as brightly colored, and you'll have more fry then you know what to do with in a matter of months lol