I haven't seen true southdown at HD for the past two summers. I think someone got wise to our use of it. or they just switched to a cheaper supply of silica because, after all, they sell it for kids sandboxes. but maybe there's something else.
Russ, I would raise the sumps on blocks, adjusted for the water line of each so that a simple 4 inch drain type bulkhead and PVC can be used as an overflow to the next, lower sump. cascade them and let gravity do the work. like the first one on two blocks high, the second on a one block layer, and third on the floor.
You will be using a pretty big return pump, pushing lots of high pressure water and burning lots of watts. so it looks like you need to use 1 1/2 - 2 inch pvc pipe. I might think the heavier walled schedule 80 pvc might be a good idea. or better yet, flexible spa type PVC?. larger pipe reduces the back pressure and increases the flow.
have you decided how many supply and return and closed loop bulkhead holes you will have drilled in the tank? it's a factor in the flow/head calculator to size your pipe and pump. I'm sure you have been trying lots of configurations.
