Hi,
I am so sorry for your loss! I do not know if you live in an area where it snows but, the water that you as using may need to be tested before you do your water change.
What does snow have to do with it? I know that here in New Jersey , if salt is applied to the roads, we see a significant rise of saline and other minerals) in the water that comes from the tap.
I don't know if you use RO water but I would start there and look at all the decor in the tank. I have found that just because it says "aquarium safe" does not mean it is for the type of fish you have. Also there are things that can be in the water that normal testing will not show( like lead). Also check the bag or bags of food you use. Pour it all out in a clear plastic bag and give it look. The smallest piece of mold can spoil the whole lot. Constipation could account for the exploding fish bellies but my guess is not enough good bacteria in the gut to help with digestion. Or it could be just the natural decomposition. Hope this helps and again my sympathy on your loss.
I do use r/o water, but I also use around 1/4 tap (treated of course) only because I found trying to formulate the proper amount of minerals to put back in was a pia. We do get a tonne of snow here. I'm going to test my tap water to see. Question though? How would one check for salt in the tap? Which test bottle would be best for that one? I don't drink the water (due to health reasons) so I haven't a clue what it's supposed to "taste" like.
I've already checked all the food and dates on the stuff in the freezer. Nothing there. The only thing I have changed in that tank is that I added plants. I can't see how that would be killing them? I'm not doing any ferts only put in two root tabs around a month ago when I added the plants. According to the instructions on the root tabs you're good to use one tab per 10 gallons. I have 80gals and only have two tabs in there. So I can't see that being it. I've done water change after water change, scrubbed filters and all media (in my healthy tanks water so not to lose any bb) taken out all deco and done the same, cleared and changed all bubblers/airline. Raked all my substrate looking for any decomposition that I may have missed. I even pulled apart my pump and cleared it too!