While it's true that you can hang out in one area at 80-90 ft, the Spiegel Grove is huge, and it's tempting to want to swim somewhere (and back). One really needs to have a plan. "Be back at the boat with 500 psi" is difficult to achieve on the fly on such a large, tempting wreck. I dove it a couple of times on a single (rental Al 80), and after the last one I decided that just doesn't give me enough gas. I use the Rock Bottom or Minimum Gas method, which is pretty conservative. For me, the Spiegel means I need a big tank (or tanks) and a real plan. Also, the unpredictable current can throw a wrench into a plan, so that should be taken into account.