Polyphosphate is a preservative used in foods to aid in maintaining freshness and color. It (trypolyphosphates) is added to many types of foods including "fresh" seafoods sold at your local grocers.
Best suggestion is presoak the foods in RO
and drain away the moisture before feeding. This will greatly reduce the amounts od PO4
introduced to the tank. You can also manufacture your own foods to control what goes into your tank. I would suggest finding a seafood store that sells live foods though.