We don't often have a choice of species but if you find one from an LSF or order from Live Aquaria you may have an idea of what you are getting. The dwarf's are small enough that you are unlikely to see them in a 55 and an O. briareus needs something a bit larger at full growth, most anything else will work well in a 55. If you can find an O. hummelincki the size is ideal but we have not seen many in the last year or so. Many O. hummelincki came from Haiti and the on-going effects of the earthquake may be much of the reason that we don't see them. We have seen none from FL in a couple of years. LA's "Indonesian" are usually well suited for a 55 but the life span has been very short (due to age). LA has recently offered some from Mexico but the species is unknown (see Bonita and Ziggy).