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All depends... I have seen a well established large pair (6" male & 5" female) be fine together in a 30 breeder. I have also seen a young group of 12 fight it out in a 75. If I were starting with a group and working towards a pair I would go with a 55 - 90 gallon. Once a pair is established you might be able to get away with a smaller tank. B. nigrodorsalis is probably the largest species in the genus.
I would go with the 29, and provide two very distinct territories in opposite corners with structure the female can get behind and be out of sight from the male. If they are well mated you may not have a problem, but if the move disrupts them... better safe than sorry.