I had almost the same size tank for 3 turts (2 WPaint, 1SPaint). I had the biggish river rocks which sounds like what you've got. Anyway how I set it up was a milkcrate on its end with a couple flat slate pieces on top for basking, which worked well because the turtles could swim underneath them. I used a fluval 404 canister and it kept the tank clean as can be. I did do water changes just because I think it's good for them, I used a python syphon...I'd stir the rocks around a bit to free up any crap between/under them, and just vac it all out with the python. Took an hour tops. I think the fluval pushes around 350gph and I had the tank maybe 2/3 full, so I was turning it over 8x.
But anyway, you can do without the rocks if you want, I doubt he'll mind, though my turts seemed to like digging around in them & moving them around.