I have a feeling your tanks have traces of ammonia in them. Changing the water once a week isn't enough at this point, I think you need to fit another water change halfway through the week.
- I hope you purchased a API style master water test kit, because if you haven't your bettas rotten fins are going to continue to get worse every week.
Also I saw those ZMed betta condos you're using in the petstore for about $8, but I didn't bother buying one because I couldn't see how you would change the water without spilling water all over the place.
Yes, they're stylish and your initial photos with your male bettas are just gorgeous. Your photos alone made me consider buying that style tank, but after getting a chance to view those betta condos in the store, there was just too many negatives.
Well designed & visually appealing, but made out of cheap materials and because of it, they scratch too easily. No lid and no pour spout. After every water change you would have to wipe the outside down VERY carefully with some sort of acrylic safe pad & a lint free microfiber cloth to avoid scratching the outside. Also you would have to clean the inside of the tank probably once a month of algae/slime buildup so you can actually see the fish.
At least with the 2.5G mini glass tanks I have that double as QT & hospital tanks, there's a built in pour spout from the one corner that makes water removal much easier.
- I would highly recommend doing water tests specifically on the ammonia 2x a week to make sure you water is SAFE.
- Take a picture of your bettas every week to see if things are getting better or worse.