This is my career, if I didn't know these things I wouldn't have a job. I have 20+ yrs of education and training to go with my experience. There was a time when I didn't know these things either... but back then, instead of just guessing, I pursued learning. Guessing would have left me unemployed very quickly. Unfortunately, most people don't get the opportunities I have found over the years, so instead of seeking professional advice, they choose to guess.
Please understand that without the proper lab work and without the ability to examine these fish hands on, no diagnosis is a guarantee. All I can offer you is my education, training, and experience... which tells me that this is not swim bladder disease based on the symptoms and description you provided.
Swim bladder disease is not something that comes and goes, which is what gives it away. Swim bladder disease will cause permanent damage that cannot be fixed... so the symptoms would not stop according to the food being offered.
When you mention that the peas don't cause the same symptom, knowing that peas are a heavy laxative, and that there are many things that can cause gas to be trapped in the digestive tract & swim bladder, my best guess is that this food is simply not agreeing with them, for whatever the reason. I can further narrow it down because all fish of the same species are being affected, but not all fish that are eating the food.
Normally Tetramin is a great food with a high nutrition content, but fish food does have a shelf life. Without a broken seal, most (not all) flake foods have a shelf life of 6 months to a year. Once the seal is broken this shelf life decreases to a matter of months. Nutritional values change once the seal is broken, and the food, once exposed to air, changes in chemical breakddown.
In the future, any flake food that is opened beyond 3 months, should be replaced.
If you see the same symptoms reappear with the changes in diet/food that you are about to make, please let me know. While the help I can provide via internet is somewhat limited, there is still informatin we can gather to further narrow down the cause of this problem.
Hang in there, don't do anything drastic, and keep close eye on the fish as these changes progress. I will do what I can to help you save them.