We are setting a gorgeous pot fountain in a flower bed on our outdoor, screened porch. It has a 4x4 plastic tank underneath it, with the pump in it. There are cement blocks set in the tank, to support the pump, and mesh over the top to support the stones that we plan to surround the pot with. The landscaper says the top of the tank should be 6 inches above the surrounding dirt to keep soil out of the tank (especially in our tropical summer rain storms). The man installing the fountain says it is necessary to bring dirt up against the sides of the tank to prevent them from collapsing. Which is correct? We are just the consumers; what do we know!!!
I would be for keeping the soil a bit below the tank. Soil in the water can wreck a pump (if you are not keeping a super close eye on the filter) and a few inches of bare tank should not lead to collapse.