I have not been able to find any info on this. All I can find is people and pets having allergies TO fish.
After medicating Lava and putting him on a pea fast I started him back on pellets. I gave him only one pellet, soaked to avoid constipation and he blew up again like a balloon!* He could not possibly be constipated that fast! I'm starting to wonder if he is allergic to something in the commercial fish food, probably wheat gluten. It's a common allergy among other pets. I've decided to try making homemade fish food for him and see if it helps. I found this site
http://www.cichlid-forum.com/articles/food_recipes.php with good homemade fish food recipes. Cichlids have almost identical nutritional needs as Bettas. I thought "The European Shrimp Mix" would be a good one to try. I'd make a much smaller portion though and I'd freeze small chunks and put them into a baggie and take one out at a time and defrost it as needed. No doubt I'd have to waste a lot of it since he can only eat a tiny portion (maybe I could feed the others with it too) but if it makes him feel better it's worth it. I like the fact that it contains peas so he could get that in every meal as well as protein. It would be very rich so I'd feed only a VERY tiny portion. There's just so many ingredients in commercial foods that I don't trust. I'm at a loss as to what else to try with him. I just want to get him well. He's feisty and active otherwise and even tries to jump for food still. If I can just get his bloating fixed and swim bladder deflated he could swim normally again...