I don't see any literature on Sodium tripolyphosphate toxicity. It might be a bit of a jump to the conclusion, not saying that it's wrong.

It's of course to late if not yet done, but a post-mortem with histology would have been the ideal means of getting answers. If Na TPphosphate was a major concern, target tissues could also be saved for testing; the pathologist would probably have to research methodology on that one.

I like the advice for dietary variety, but also like to suggest live fish if a good captive bred source can be found and/or a relevant deworming protocol being in place. If I'm not mistaken, thiamine deficiency is primarily a concern with frozen fish.

Ian