Sounds like 'old tank syndrome'. I remember reading a few posts over in Shimek's forum on RC about old sandbeds. I believe he was saying that after several years accumulated detritus and bound-up nutrients can start to cause problems. He doesnt recommend re-using old sand.
I guess according to his info, one should tear down the tank once every several years and clean or replace a portion of the DSB. I can't wait...
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