I am thinking of ordering this...does anyone think its what I need...has anyone used it?
http://www.amazon.com/Pentair-Aquatics-Pond-Eater-Brush/dp/B00545B1PO

Is your pump sitting on the bottom? I'd recommend lifting it up so it isn't sitting in the muck.

Why a sand bottom?

Topsie, your system of filtration is part of the problem. Another part is the sand on the bottom. Sand may do just fine in a natural unlined pond where there is a constant flow of new water but that is a completely different animal than the closed system of a lined pond.

Your pump is simply recirculating the gunk that gets into the water over and over, through the filter material and through the pump into the waterfall which stirs up the sand and debris which gets pulled through the pump and though the waterfall which stirs it up all over again. This is probably breaking up bits of debris making the suspended particulates even smaller. They just never go away.

To see just how much is going through that cycle, fill a clear container with the pond water and let it sit for a couple of hours. Sand and particulates will settle out.

The "filter'' system you have may collect some of the particulates and sand but the majority just passes right on through. Even if you clean it every day the water will just continue to get more and more dirty because you are pulling the dirty water into the pump and recirculating it.

You would do better to put your oh so very heavy lava rock that is so hard to clean and the quilt batting in the waterfall tank where particulates will have a chance to settle and be trapped rather than recirculated. (You can tell I'm not particularly impressed with lava rock but others like it) If you can make the water swirl before it leaves the tank, the filter material works better. The plastic screen may do for a prefilter to prevent larger debris from damaging the pump. The sand is not doing the pump any good since it is abrasive.

I don't think this device will do anything for you except make your wallet a lot lighter. Did you read the reviews?

My pump sits in a surface skimmer box behind a coarse filter wall. In spite of this, the muck from my pond still gets in the pump well. It only is disturbed when I do pump screen cleaning. When I do my pond semi-annual cleaning, I vacuum out all the muck with my Oasis. I could do it more ofter, I suppose, but it really isn't a major problem. I figure the muck also acts as an anaerobic area to denitrify any nitrates. ( see http://www.koipondfever.com/t-koi-pond-filtration.aspx)