The metal braces under the coroplast belly do not support the tanks, just the belly material. Last year, I had a leak in my grey tank, and after making the repair, I added two additional angle iron braces, so there would be a brace every foot under the tanks, and then installed blocking under the tanks so they would have support from the braces. The tanks have a slope in the bottom, 1/2" per foot so they will drain toward the drain valve. The photo below shows where I glued blocks of styrofoam and a 2 X 4 in one position above the coroplast to the tanks and above the braces. The tanks do not sag now, even when full.
