I recently turned an old wooden barrel into a small pond/water feature in my garden. There is a pump/filter and aerator installed and it has been going for almost 2 months. I have no plans to ever add fish or inverts but want to incorporate a few plants. Right now i just have some anacharis floating around. My problem is i keep getting this slimy gray coating on the walls and bottom of the barrel. I've used an algae killer and water clarifier but neither makes a difference. Most recently (2 weeks) i totally emptied the barrel and scrubbed out the goo then refilled. Now it's back again... The location is a mostly sunny spot and the water is about 18" deep. Any ideas or help to fix this problem would be greatly appreciated, thanks!