I'd been having some problems with evaporation with my horizontal scrubber. It had been set up since the spring and had worked well but as the weather has got colder here I've been having problems with condensation at night which culminated in my getting a mild electric shock from the lights a couple of weeks ago.
So I broke it down and placed the tile screens in the sump. with three CFLs over them and a large power head circulating water around them. I wasn't expecting them to do much filtering, just to keep the screens alive while I built a new vertical scrubber to replace the sump.
Anyhow, I got busy and I wasn't able to build a new scrubber in that first week and after 7 days I had a look and found that the screens had grown as much algae in that week as they had on the old horizontal scrubber. So I tested the water, which was fine, cleaned the screens and left it another week.
Today I cleaned the screens again and there was more algae than I was getting with the old system. Tested the water again and the tests still show zero nitrate & phosphate. So I've cleaned again and left it.
The screens are completely submerged and the the only flow is being provided by the powerhead. The lights (3 x 18w CFLs) are six to ten inches away from the screens.
So I'm going to let it run and see what happens.