I would say the "standard" 24 inch isgood for full grown. 18" is pushing it. I say this becasue lower water lvls seem to stress them a bit imo. Possibly seem to be more suceptible to predation?? I dunno. I would'nt go under 18
With that length I think it should be ok. In my 30" high they do wander all the way up from time to time but usually only when feeding. 90% of the time they are around the bottom.....
thats what i wanted to know cuz mine stay down at the bottom even when feeding so i wanted to no if the tank sould be short lol. how tall are full grown 12 in reds anyway?