Mind you I'm just suggesting things I've seen in puffers after about the one year mark, as with this hobby there are many variables, and sometimes you just don't know.
Feeding fiddler crabs, get a small one and drop him in live. Sometimes the puffer may be shy if gotten used to being fed dead, but most go crazy, they love the hunt. Live food will contain nutrients that dead food does not and the shell helps with managing the puffer's teeth. the live food isn't absolute, it's just a suggestion, try the ghost shrimp. I'm just a strong live food advocate because I have seen the marked difference in health with many species afforded an inclusive of live foods. Puffers can live a very long life if fed the proper dead foods and live food really isn't necessary. Same said, the shrimp you feed should be shell on, the shell has concentrated nutrients like amino acids and minerals including ca and will help with managing the teeth. Many will freeze their fresh food from the market because of fear of parasites, I think this would address your concern with the mussels, you can also open them yourself; or get the Gamma Fish variety from your lfs. Fish like puffers do seem to effected by poor nutrition and paying attention to this is a good idea. Maintaining reef levels of mg is also a good idea as puffers also seem to occasionally have digestion issues. Mg has been known to help with digestion.
At this time you also see issues with puffers that have been treated with copper, antibiotics, ich x, or exposed to things like tank cleaners.
If your tank conditions work out and there are no bullies, these 2 issues are ones that I have seen. The poisoning issues is not something that can be fixed, it is only hoping for the best if that has been the case.