from 'seriously fish' - "Considering it can reach almost two feet in length a tank with a footprint of at least 84" x 30"/210cm x 75cm is advisable. Height is also important due to this species' tendency to swim with an oblique, head-down posture. 24"/60cm is perhaps an adequate minimum. It's worth noting that a tank with these dimensions would hold almost 950 litres of water."
I think if you could get an adult - known to be fully grown at 12-14" then it would be ok - just but I still think it is an absolute minimum space for a fish that size and I know that personally I wouldn't be happy with keeping one in an 18" wide/deep tank.
Yes clarias can be aggressive - mine is aggressive only to other bottom dwellers though and this can be a trait many catfish share.