I'm just curious but is this your first turtle? Because these guys are not for beginners and can turn you off turtles real fast. Just so you know that 30 gallon tank won't even last a year. The 75 maybe 2 years. Plan on something around 600-800 gallons for life. A pond is the best choice for an adult.
I've never kept this species of snapper but I know they like to feel invisible in their tanks so provide plenty of driftwood, logs, large smooth river stones and live aquatic plants. I think gravel will just be a pain to clean because these guys are very messy and plan on doing a 50-100% water change once a week. You're also going to want to invest in some kind of powerful filtration system. The water shouldn't be too deep, just enough for the turtle to comfortably reach it's head to the surface to breath. On a plus side, unless you're breeding them you don't have to provide an area for them to get out of the water.