You can leave your Rhom in your 30 for quite a loooong time. As you've read, Rhoms are slow growers. Your 3"er can be more than happy to have enpugh room all to himself. I have a 5" Rhom in a 30 gal and getting a 7-8"er in another 30 gal. No need to get a bigger tank just for 1 fish.. yah feeel?!?!?