The tank that measures 36L X 18W X 12H is a 30 gallon breeder. The 40 gallon breeder has the same 36 X 18 floor space but it is about 14-15 inches tall. The tank you are planning on using (48L X 12W) will have approximately the same amount of floor space as the other 2. The main problem I would forsee is the height. It seems that BP's tend to stress a little when there is more than about 15 inches of height. If you decide to use your longer tank, I would suggest putting stuff like fake vines and stuff to fill in some of the dead space.
Depending on which brand of UTH you get will determine the size of it, but I wouldn't go any bigger than a 11 X 17 for either tank. That would cover almost half of the 36 X 18's and a little more than a third of your 4 foot tank...
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