That of course is the dilema of the big fish. I often see people posting that they are going to buy a fish and right now have no way to house it at its full size, but then proclaim to be able to buy a bigger tank in the future. The problems with that are
1. Quite often that bigger tank never comes (and I am glad I didn't jump the gun on the rosey barbs I want to get)
2. By the time it looks like the fish will need a new tank, quite often the fish has been stunted and will never attain its full size or life span.
There are always good choices for the tanks we have so try and make sure tp get the fish that you will definately be able to take care of.