I am pondering a similar idea for my pond. I was thinking about making a several hundred gallon shallow (2ft) pond ten to fifteen feet from the high water mark on the big pond. Pump water from the main pond into the small holding tank and have the overflow trickle back down into the main pond via small stream and cascade. Figured it might be a cool sanctuary to raise some freshwater shrimp, crawfish, minnows, etc with out the BG and Bass getting them before they can grow or reproduce. Any comments or suggestions?