While a 20 long may be preferrable for apistos, I think a 20 high can work. I have a breeding pair of Strawberry apistos in a 15 gallon(same footprint). It may cost more, but I would recommend starting with a pair of apistos. Also go with the "tank-raised" varieties rather than the wild ones, because they tend to be hardier.I would think apistos would also be an option, even though a 20L would be more ideal than a 20H.