k, Ive had these tanks set up for over 6 months, they each have a ugf as well as additional filtration. My 20 gallon stand has an opening, i assume so you can see the underside of te tank....In the beginning i would check the bottom of that one at each water change and it always was fine, clean, etc. i stopped looking under there until last night, and eek! there was a layer of fine poops and stuff underthere. I dont think it was TOO much, I could still see through some parts, but how do I clean this out? This must be why people dont like undergravels....Id almost prefer to remove them, but thats so much hassle....