smileyface9...out of curiosity, were you running your unsuccesful UGF with air power, or a powerhead?
I ran a UGF on a 29 gallon tank for years. This particular tank was memorable for me because it was on a wrought iron stand that was open under the tank. If you knelt down on the ground and looked up, you could see the bottom of the tank. I always had a large powerhead on this UGF, and the underside of the plate was a clean as a whistle. After several years, there was a small bit of brown crud in the two corners furthest from the powerhead, but maybe 4 square inches each, max.