In a tank that size, not much. Honestly a pair of bleekeri should be in no less then a 75g tank because they can grow to 12" and will need the extra width to turn. I've had success keeping Pictus Cats, Pleco's, Convicts, Green Terrors and JD's with them and in a large enough tank you can keep most New World cichlids including Oscars, Sevrums etc. Are your pair breeding? If so then I wouldnt try anything other then maybe a pleco.