If you want to breed all 6 then you're best off going with 3 40breeders or 55g's IMO. (Not an expert, but to get the same footprints as the 3 smaller tanks, the single larger tank would have to be at least 9 feet long, which is rather expensive) A 125g divided could work for 2 pairs of Jacks, and then a third pair could go into a 40/55. Personally I wouldn't keep a pair of JD's in a 40breeder, but a couple places online say it's acceptable. (Not really optimal though). You could do 2 125 gallons, divide both tanks so that you esssentially have 4 3'X2' compartments, 3 compartments for the parents, and 1 could be split up further for fry.