I've purchased a 125g setup off Craigslist that came with a Regal tang. I know the tang guidelines indicate a 180 gallon minimum for this fish (no biggie gives me an excuse to setup a bigger tank in the basement). What are everyone's thoughts on what size will necessitate a space upgrade? Currently the fish is about 5" and seems to be thriving (take that with a grain of salt, I've had this setup for a grand total of 2 days).