Two contain gravel but with different drains. One is DWC and the fourth is expanded shale.
The ABS pipe running along the back ties both IBCs together and provides effluent to the growbeds. The pipe run includes several removable plugs for both cleaning and expansion. I used ABS to help keep algae in the pipes to a minimum.
I also placed the IBCs in the corners in order to utilize their weight in keeping a poly roof connected to the asphalt. The closest tank is about 2" lower than the farther one and I will have to correct that in order to draw from both tanks equally.