The main problem with inbreeding any sort of fish/animal is that you can, and probably will eventually, tend to increase the liklihood of any unwanted gene or trait to manifest itself i.e. curvature of the spine or somesuch genetic deformity. You could probably get away with it for 1 generation but personally, I wouldn't really push it past that unless you were working with extremely rare and endangered fish that was impossible to find more stock of.
Which Geophagine are you working with at the moment?
A.N.G.F.A. North American Co-ordinator