Without a target turnover rate there is no way of making a recommendation. On tanks this large most people use two return pumps for redundancy. When choosing a return pump there’s a few things to consider, head loss, pump curve, and plumbing fittings. Let’s say your target turnover is 4000gph. When a return pump says it’s 4000gph that’s at zero head height. You can basically cut that in half after factoring in head height and 90’s. You will also be running the pump at 100% duty to achieve the 2000gph. That will shorten the life span of the pump. In this example you would need two 4000pgh pumps to hit the target of 4000 gph.