Preservitives is exactly correct. Even in flake food. How do you avoid it. Even making your own food you would have to find out somehow if the processing from the seafood you purchase has been treated in any way with a presvertive, most likely it has.
But on the whole you can make your own and rinse the heck out of it with good ole RO and probably do better then other frozen food and wind up saving money as well.
Phosphates can also come from decaying matter which is what frozen food can or is becoming.
Here is a small phosphate list that are used as preservitives.
Acetylated distarch phosphate (thickener,
stabiliser)
Ammonium phosphates (acidity regulator)
Bone phosphate (anti-caking agent, emulsifier)
Calcium phosphates (acidity regulator, emulsifier,
stabiliser, anti-caking agent)
Distarch phosphate (thickener, stabiliser)
Hydroxypropyl distarch phosphate (thickener,
stabiliser)
Magnesium phosphates (acidity regulator, anti-
caking agent)
Monostarch phosphate (thickener, stabiliser)
Phosphated distarch phosphate (thickener,
stabiliser)
Potassium phosphates (acidity regulator,
emulsifier, stabiliser)
Potassium polymetaphosphate or Sodium
metaphosphate, insoluble or Sodium
polyphosphates, glassy (emulsifier, stabiliser)
Potassium pyrophosphate or Sodium acid
pyrophosphate or Sodium pyrophosphate
(emulsifiers, acidity regulators, stabilisers)
Potassium tripolyphosphate or Sodium
tripolyphosphate (acidity regulator)
Riboflavin or Riboflavin 5'-phosphate sodium
(colour)
Sodium aluminium phosphate (acidity regulator,
emulsifier)
Sodium phosphates (acidity regulator, emulsifier,
stabiliser)