If this 55g tank is minimum 4 feet (120 cm) in length, you could have a group of Congo Tetra. This is a shoaling fish (= needs a group) and can be quite active at times, so a group of 7-9 would be best. Male and female are easy to distinguish, and I have always found it best to have something like 4 males and 3 females, or five males and four females. Males will interact among themselves much more often than they do with females, though this may partly be due to the presence of the females. This is a peaceful species but you need to avoid any fin nippers as tankmates because the extended filaments on the caudal fin especially are easy targets.
Glass catfish is another matter. This is a very delicate and sensitive species, and it needs a lot of cover, not an open tank. I would suggest this species is best in smaller tanks where it can be alone in a good sized group (it is also shoaling) and not stressed out by active fish. The Congo Tetra running their "relay" races down the tank as they do would unsettle the Glass Catfish.
