mtnbiker44, not sure of your set up but a deep sand bed can be defined as 3 inches or better. Yes your P04 is very high. Mine was high also and I did two months of weekly water changes and it did not help. It was not until I paid a professional to come check out my tank and was told how to clean it, after that I got results. Probably the best money I ever spent. My Sand bed is three inches and I had NO trouble at all doing a deep cleaning. He had me clean it twice a week for three weeks and my tank has never looked better. I had cyano all on the back glass and rock and top of the sand bed. Water changes are great but if you have lots of detritus in your rock and sand without cleaning , water changes are not going to get your P04 where it needs to be, 0.03 is a good goal to shoot for. Man I"ve been there and it can get frustrating and your tank will have a hard time, if you've got corals or anemones. Maybe talk to your LFS and see if they have a name of a good maintenance person that can come out and look at your tank and give you some hand on ideas, best $50.00 I ever spent. Now my tank has been running for several months with a 0.03 PO4, and 0 Nitrates. I have no mechanical filtration only LR, Live Sand, a fuge. I've taken out over time all the bio balls, nitrate factory, Sock filters and all sponges. The tank runs itself if you feed it right. That would be my best advice and keep us posted...
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Not the best picture but you can see how clean everything looks. Before I'd spend hours cleaning out the cyano and by that afternoon it would look the same. This is two weeks and I've not even cleaned off the glass. Best of luck.