New S. Rhom....

This isn't much of a question... just a post, I bought a 2 inch s. rhom from shark aqua about 2 weeks ago and he is crazy. Very aggresive, at first he was kinda shy. But about a week ago he started attacking whatever I put in there. One night I put my hand in there to move a plant and it looked like he was gonna come over and try a bite. So I grabbed a net to nudge something over and he came at the net, and bit a hole in the netting! He isn't as fun as watching a group of red bellies but he sure is gonna be a handfull in about 10 years! I can't wait. I do have a question too, I have him in a 15 gallon now, I figure that will work fine till he is about 4 or so inches, then I plan on putting him in my 29 gallon tank (30Lx12Wx18T...) how long in that tank?? I have a 75 gallon im gonna use down the road for him(48x18x22), is this good for life or should I get a 125 for him and skip my 75 and sell that?? Thanks guys!