Sodium tripolyphosphate is a preservative commonly used in seafood.  The amount that can be used in foods is often regulated, not because it is considered at all unsafe, but because it is very efficient at retaining moisture content and can thus artificially inflate the weight of foods in which it is used.

Sodium tripolyphosphate has been approved for use in foods by the Food & Drug Administration, and I'm not aware of any controlled study suggesting that it is any less safe than any other sodium-based food ingredient, like common table salt.  ...But it is a sodium-based chemical, and with that comes some related implications: it being a potential irritant to some sensitive consumers, contributing to blood pressure issues, etc.

Personally, I avoid such things by rarely eating processed seafoods, instead favoring to catch my own from wild stocks.  I realize that option isn't available to everybody.  When shopping, just be aware of ingredients in whatever you buy and, if it is of concern to you (and it probably should be), your consumption of sodium salts.

The more problematic use of sodium tripolyphosphate may be in detergents that often ultimately find themselves washing down the drain as a postlude to use.  In containing phosphates, sodium tripolyphosphate can contribute to the eutrophication of surface waters.


Best,
Eugene