STPP isn't the best for peeps or fish. It is actually a suspected neurotoxin according to NIOSH. What it's generally used for is to make fish that is at the end of its "expiration" look fresher and more glossy and to reduce "thaw drip".
If you have the option, I wouldn't use it, and I definitely wouldn't use it long-term if you can help it. That being said, most of the frozen shrimp on the market probably contain it to some degree, as it's currently OK'd by the FDA.
HTH