I think the average captive RTC would be fortunate to have that set up. If you can make it 5' wide, all the better. It would need some serious filtration though.
As quoted in the catfish thread on this site, minimum size for a large RTC or TSN is 10 foot by 4 foot. Depth is a lot less important, 2-3 foot should work. IMHO. Also TSN is a lot more active, factor that in if going for one of those. Good luck!
Chhers,
Ken.
id personally go with at least 5 goot depth considering ive seen some TSNs (though not the species commonly seen in the hobby) that rival the size and thickness of pimas. and TSNs are much more stiff than RTC. so a 36" red tail could turn around in 24" of space but a 36" TSN could turn in 30" depth. see what im saying?
and also this is all based on the facts that you want to keep them in that for LIFE...