At that depth most predators can't get to the fish anyway. I don't recommend cinder blocks, Even when well seasoned they can be quite abrasive. I've seen many a long tail Goldfish and Koi get split when there were cinder blocks in the pond. I can't say they were the exact cause, But there are many other potions. Milk crates are one, they can be put in as they are and are great hide outs for smaller fish to retreat, or they can be modified so that larger fish can use them as well. I use them and stack rock on top to make a long shelf. In the Koi only pond I use 8- 10" black drain pipe(Blends very well on the black liner so almost impossible to see from the surface) cut into 36" sections, Tied them together and placed a large flat rock on them to hold them in place. Very seldom do any of the larger fish use it but the smaller fish are almost always asleep in them when we turn the lights off at night. Another suggestion I use this spring, on ebay there is a floating plant net that is about 30" across and floats. Put a piece of foam in the middle and use it as a floating Island, many plant choices but plants are just the beginning, Use your Imagination and the possibilities could be endless.
