String algea --- let me know if you come up with an answer that doesn't cost an arm and a leg and involve chemicals.
Yes, I have tried the bentonite clay/kitty litter also.

Yep sounds like string algae. Ponderpaul I struggled with string algae for years. I did plenty of water changes, kept the pond crystal clean and still had problems with it. The only thing that has worked is veggie filtration. I keep lots of watercress in the pond and filters and various other plants in the settling pond. Once I added this the string algae problem resolved. Also I cut back on 10% weekly water changes as my source water is rich in phosphates. Hope that helps.

Lisa

Guess what? It's definitely not string algae. I have that too and can control that easily. This other stuff is a blackish green substance that is loose on the bottom and rises to the top in warm weather and gets caught in my skimmer filter. It doesn't look anything like the string algae. I can't be the only one with this!

Lee

Is it slimy and form mats. It sort of sounds like blue/green algae.

My guess it is what most people call pond scum. Grows on the bottom of the pond and small globs pop to the surface in the heat of the afternoon and is drawn into your skimmer. Each morning the pond is clear only to repeat once the sun warms the water. I have a problem with it this time of the year when the bare root plants have not taken off enough to out compete this form of algae. If it gets too intense for my liking I vacume the pond but it solves itself with the plants growing in by mid June here in Ohio. See it every day in natural ponds if not treated with copper sulfate. Grass carp don't eat it BTY.

Frogman- That about describes it, but is there another name for it? I'm looking for ways of controlling it. I know the fish won't eat it.

i have the exact same junk in my pond every year. i also have some string algae but it doesn't make the mess that whatever this other stuff is. the crud on the bottom will suspend with just the slightest disturbance and make a huge mess in addition to the mess that floats up. i did read a post on here a while back that the person added a lot of aeration and that helped it alot. a bubble stone or something i think. i have a small waterfall and a fountain in mine but it hasn't helped. i resorted to the hard stuff when it got really bad and used some algae fix stuff. it worked for a while but came back in a few weeks. good luck with it and be sure to post back if you find a solution.