If he was in a 10g tank, well, first I can't believe that he grew to a length of 12", unless he was kept in another tank and they moved him to the 10g to seperate from the other fish. Depending on his lifespan in the previous tank, he may be done growing or near maxed or has a very slow growth rate. There are plenty of people that succesfully keep Floridas in a 72x18 footprint. I also agree with not buying a fish that you currently house for life, but if you do plan to upgrade, anything is better than a 10g tank. Also, you will love the fish and anyone that visits will be amazed. They are definantly an awesome conversation piece.