I agree that something isn't right with your DSB. I would get some DSB critter packs and go from there. The reason your tests show nothing is because the diatoms and cyano are consumming the PO4 and the nitrates. A fighting conch will help keep the bed clean, but I wouldn't just rely on the conch though. You'll need the little critters to clean the sand bed.
I had also had a problem with hair algae in my 40 gal. I purchased about 10 red Scarlet reef crabs and they devoured it. Now i'm trying to get rid of my Cyno problem.....
I agree with Meloco14, IPSF was were I got my sand critters. Gerald is the guy that you deal with. Very quick response and professional. He even responded to an order I placed on a Sun... as in that day. Service and product is great. Got similar package as mentioned above. You can also get a bacteria recharge pack w/ aerobic/anaerobic. Rob_Reef, you can definitely recharge a sand bed any time. I imagine that due to it's age it must have some activity already. They even sell packages especially for that. Next time I'm going with the "Surf Zone" Live Sand kit. Very complete and complimentary to what I've already added. BTW I got extras of many items, that never hurts either.
Indo Pacific Sea Farms check http://www.ipsf.com They are in Hawaii and have a facility right on the ocean. For me in CA they are a great source of sand bed critters, as well as breeding kits for amphipods and such which can be used in a tank or refugium. I think they also have six different types of macroalgae which are sent ready to drop in refugia for your herbivores like tangs, although I have no personal experience w/the latter. The site is basically all text, but not bad and only one page. You just have to sift through testimonials between product areas. Hope this helps, I'll be ordering soon myself.
Well, I just got home from a 5 day vacation and the tank really went south.
The cyano has covered 3/4 of the tank and the hair algae as spread even more. I think something in the biological system broke down and now has crashed the tank.
What would the best route be for a tank crash like this?
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Stir up the sand and do massive water changes over the next two weeks. Remove the rocks and scrub them clean, using salt water from the tank(don't use freshwater).
Oh yeah, tangs will eat hair algae(sometimes) and so will blennies. However, neither will eat cyano. For cyano you will need snails and/or cukes. Syphoning cyno helps as well.
Smart move. You'll get the exact same nitrification/denitrification benefit from a shallow sand bed without the drawbacks and maintenance requirements of a DSB. Switching to 20K would reduce the likelihood of another outbreak, also. Drawback = NONE.