I agree with Ryan that any central American pair can turn on eachother in too small of a tank. I've kept lots of cryptoheros species, and have lost my share of females to aggression. Dividers will work, but aren't the best looking thing in a display tank. I've recently been experimenting with keeping a group in a 55, specifically with cryptoheros coatepeque, an uncommon type from the convict complex. The group is 2 males and 4 females, with a few other cichlids to draw the attention of pairs when they breed. I've had this set up for close to a year and have not yet lost a fish to aggression. Each male has his territory on opposite sides of the tank and they each breed regularly. I have to siphon out fry if I want any to survive, but so far it is working well.
In a 20, I would always have a divider handy. HRP's are a good choice, as they are some of the least aggressive cryptoheros I have kept. If you are looking for something really out of the ordinary, you could try your hand with ivanocara. Your initial outlay for the fish will be pricey, but you wouldn't have any issue moving out fry at a good price.
I may try this method of siphoning fry out into a bucket and then removing them to their own tank.
I love the blue of the HRPs. Aggression may be my biggest obstacle. If I were to house a pair in a 20 (or I have a 40 if a 20 is too small), what dithers/target fish would be best to give it a community feel that will also keep the pair from killing each other?
I love the look of the ivanacara too.
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