Discus breeders often use 29 gallons for their pairs since the tanks are very cheap, but these tanks are kept bare except a breeding cone and are usually tied into a plumming system that overturns the tank water several times a day. Or they do massive daily water changes.
Spot on dwarf pike. But,...in my expirience the minimum size for a proven pair of discus is a 35 preferable cubic tank. The breeders do use 29's but they don't tell they pull the fry or eggs asap and artificially raise them. Feeding bbs every 2 hours is a serious bioload and in a 29 this can easely lead to poor water qualety.
When a pair doesn't breed they often have serious quarrels. Males can hares the females when the male is ready to breed but the female not. If the tank is to small the female can't avoid the male and the male can even heras the female to death. Something quit common in cichlids. Some people tend to forget Discus are cichlids as well. Mild mannered for most of the time dough.