So, your nitrates and phosphates have been OK up untill the last couple of monthes? Have you always had enough flow to keep detritus from settling on your sandbed? Any missing livestock? (sorry I had to ask) One thing you could try is siphoning off the top 1/2" of your sand and replacing it with clean sand (maybe a bit more to increase the total depth) Seems hard to believe your sandbed would be at the end of it's life already.
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