First off I haven't kept discus yet, I'm working on getting some for my 90 but from what I understand a 29 just wouldn't work because there isn't enough space. A good rule of thumb is about 10 gallons per discus and since they are a schooling fish you really shouldn't have any less then 5, making a 55 gallon the smallest tank you'd want to put discus in. Plus due to them being fairly sensitive to water quality the smaller volume of the tank would cause the water to go fowl quicker meaning you'd have to do water changes maybe every couple days just to make sure the water is clean. But again I am not an expert, these are just things that I have learned in my quest for discus. I hope I've helped you out, and maybe someone will reaffirm what I've said, or maybe correct me if I'm wrong.