how certain are you of your phosphate test kit? it could be that you're not really at 5ppm, and you're not adding any new in, in turn leaving your plants desiring more phosphate. you're true phosphate level might be near 0ppm. however, *don't* just assume this is right and dump some PO4 in until you double check with another test kit from a friend or at LFS (most good ones will test your water for free, just take them a sample from the tank)
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