Necrocanis had a 3ft+ rtc x tsn and a bunch of other larger cats in a 300g stock tank for quite a while. So sure you could do it but once they get 3ft plus they would need something bigger. Where did you get your stock tank?
btw, I have that 28 gallon if you still want it....
I used a rubbermaid 300 gal as a pond until I dug a true pond.
I kept Tilapia in it, and I agree with the poster who said, visually its not that great, and its really not that big when you think about the bodies of water large fish come from, its kind of the size of a lagoon just before the lagoon dries up, a large puddle.
I do have a smaller one I keep Paretroplus in, mostly to give them a better sense of security for breeding, as opposed to glass walls.