Sorry to hear about that ;-( don't lose hope yet.
What I would recommend if you really think it's the tank:
1. Take all the gravel out, and put it in one of those pasta things that can help drain it. Then take everything out and put it on top of the gravel, next get some vinegar and just pour it in and rinse it with vinegar, if you don't have vinegar scrub with soap and make sure there is no more left-over soap.
2. Rinse the gravel with hot water & vinegar, if again you don't have vinegar squirt some soap on the gravel and then just lift your gravel and just use your hands to swish it around. Or if you have one of those tabs where you can adjust it so it's like a powerful sprinkler and you can pull it out like a hose use that, if you don't let the hot water run for about 2 - 5 minutes (8 maximum) and just let it get rinsed off. (Do it with the same with plastic plants, but with the plastic plants hold the under the tap and just squeeze them for 3 - 5 seconds and just shake the water off and lay them on a paper towel or something. (If the gravel is still 'smelly' or makes the water cloudy, try buying a new pack of gravel and rinse that for 5 - 8 minutes in hot water & it'll be better.)
3. If you have a filter, just rinse the bio-bag out or replace it with a bran new one, and if you have let's say a "Tetra Whisper 10i" filter just rinse off the body of the filter not the bio-bag (hot water)
4. Put everything you washed back in the tank (make sure you rinsed of the tank too!) and then fill the tank up and then treat the water with fish water conditioner (like aqua plus) of course! And then plug in the filter and just let the filter run for 24 hours so everything in the tank just has time to clear up and just stay clean.
5. Cycle the tank, it will help, and make sure your testing your water! (If you cannot, just hope for the best that all the levels are good)
6. After all of that including a cycle GO TO A DIFFERENT STORE when you buy your bettas, it could be the store your getting your bettas from, try a new store or a local pet store which isn't hosted by 'petsmart' or 'petco' but if those are the only ones, try a different one near your area. Or try to bid on beautiful bettas on Aquabid.com!
7. Best of luck!
(If you ask me "Why rinse out with vinegar? Well my friend had his females dying randomly & from dropsy, so he once day poured some vinegar in the tank and rinsed then scrubbed and then added hot water and emptied out the tank, he did this 2 times. Afterwards he did the same thing with the gravel & conditioner, then he let his tank cycle & run for 24 hours filter and then he never experiences dropsy again, and all his fish are amazingly healthy the tank is very beautiful & lovely planted)