I am looking for about 6 more cichlids and a bottom feeder" Getting 6 Central American cichlids into a 55g has two (actually three) problems. Maximum adult size of the species you want,is it compatible with Convicts,and will it "cross-breed" with Convicts. As far as a peaceful species that will get along with female Convicts (possibly) there's Herotilapia multispinosa http://www.borstein.com/sam/images/herot... the Rainbow Cichlid.
A pair will work fine in that volume tank. A single Salvini http://cameron-digital.com/coppermine/al... OR Carpinte http://forum.aquamojo.com/uploads/122307... would also work. Shooting for 6 CA cichlids in 55g could be a disaster if not choosing your species wisely.... looking through the list above I can't believe anyone would suggest an Amphilophus species (which would OWN a 55g by itself and take out anything else) or blindly throwing out Archocentrus (or Amatitlania) species .... one which IS the Convict... that can "hybridize" with Convicts, as suggestions.
You could probably work in a Thorichthys species like Fiemouth also. Suggestion for your cichlids .... current two female Convicts 1 Firemouth 1 pair of Rainbows (which wouldn't be too bad even if they spawned) ... and that's it,unless you plan to upgrade to a larger tank. Then possibly either the Savini or Carpinte.
Far as a "bottom feeder",I'd stay with either Bristlenose or Rubbernose Pleco... but a Hoplo,Rafael,or one of the midsize Pimelodus catfish would work.
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