The 75 is a bit big for him, if he's under Four Feet in length, I'd downgrade to a 40 gallon terrarium tank in a heart beat.
Once that's accomplished, start thinking about all your variables for your environment and humidity.
I know exactly what you mean when you say salt water is an expensive hobby, that's our bread and butter, and boy is it some kinda wild hobby. You'd do well to sell those large tanks you have, especially if they're for fish. Fish tanks are drastically more expensive than terrarium tanks, you can probably recoup a great deal of capital by doing so.