It is common for a Betta to get a big tummy after a feed - I used to think this was bloating myself but it isn't. Bloating is just a huge tummy that won't go away at all, this can indicate internal issues like parasites.
If he has been bloated for a long time, try fasting and ceasing AQ salt for now. If it still doesn't go away you will want to treat with Epsom salts. at 1tsp per g. But I suspect he may just be full from noms.
If it is a new tank it should be ok. But I had a local fish store that commonly used products and just repacked them as new. I bought a tank from him once and only realised it was used by some slime and scratches along the bottom. But if it was a reputable retailer, I doubt this would be the case - the LFS I went to was a dying mum and pop corner shop.
Disgressing, I couldn't say with certainty at this point mate what is the sole cause or contributors to fishies deaths. Maybe change up your spectrum, different food, different conditioner.
Ultimately it's good you're trying, generally speaking most who come here provide a better life for a Betta than others who don't. So in a way if we can't pinpoint a specific cause for certain, we usually just slot this under the best treatment before they went on to the big puddle in the sky.
Keep trying, troubleshooting issues are a pain when we don't have certainty. But as I said, try changing your conditioner and food.
Which made me think just then, did you feed the same stock of bloodworms to the others?