anyone ever used a old hot tub water pump to circulate large tanks?

Hmmm, not designed for aquarium use or saltwater.
This sounds a bit... penny wise and pound foolish.
Plenty of good products available for this use.

Are hope you're not serious. Are we talking about the giant tank in your garage here? You are going to need thousands and thousands of dollars to do this properly and the last thing you want to skimp on is the heart of the system... don't mean to be rude but if you are serious, I think you need to consult a pro, develop a realistic cost profile and think this through first.