Well, I hope you're getting the phosgard at J&L, or some other poular place. I see it's about $30 per 2 liter container, not sure about weight.
The canister filter can act as a sink for nutrients if the media isn't changed often. I think Randy Holmes Farley had a recent article on phosphate removers, might be worth looking at. I'm sure you know that a major source of PO4 is the food, with frozen food containing juices being the worst. I always rinse any frozen fod I feed to help remove phosphates.
Do you have any live rock in the tank? Crushed coral can hold a fair amount of detritus, which in turn can hold (undetected) phosphates. Also, if your bulbs are really old, it's thought they can help trigger the growth of PO4.
What is your water change schedule? And what is the source of your water? If you don't have an RO unit, it might be worth it to you to buy water from the grocery store. You might not need the expensive seachem stuff. Let's see what else others can add to this, since my area of specialty isn't algae/cyano problems!
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Brad
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