RW-4 in a 20G

[bleevin]

I have a rw-4 in a 20g long. I can't seem to run it in else mode. It produces a lot of undertow? and slowly clears the sand under the pump. So far the only way I can run it, is in W1 mode.

I have about an inch of sand and the pump is about 2 1/2 inches below the water line. The water line is about 0.25 inchs below the bottom of the black trim.