A 55 l tank would be a bit boring with just the one knight goby, I fear. These gobies are somewhat active, midwater fish and they look most attractive in groups.
With 55 l at your disposal, your best bet would be to look for some of the smaller gobies, and then perhaps add some small livebearers. For example, even if bumblebees don't appeal, you could opt for rhinohorn gobies (Redigobius balteatus), desert gobies (Chlamydogobius eremius), or one of the various Rhinogobius species appearing in shops such as Wildwoods. Combine with some nerite snails and some algae-eating shrimps, and then for the top of the tank, either wrestling halfbeaks or a livebearing fish such as Endler guppies, Micropoecilia picta or Heterandria formosa could be used to add some activity. At a pinch, even Limia nigrofasciata could be used, though they're a little big for this sized tank. All of these fish would do fine around SG 1.003 to 1.005, so you could decorate with bogwood and clumps of Anubias, Java moss and Java fern.
Cheers, Neale
