I still think you have traces of something in either your gravel or water. It could also be minerals/chemicals in your hot water you're not getting out when you're rinsing the tank.
Like I said with the 1G betta condos & 2G betta houses. They're too light to be sprayed with a detachable showerhead. Basically you have to hold onto it while you're cleaning it.
I tip my 2.5G glass tank on it's side and spray it down if it's really nasty. It's just heavy enough so I don't have to worry about it moving around.
You don't have that option with your ZM acrylic tank because it will tumble around your tub if you don't physically hold onto it with 1 hand.
- Ammonia from not enough water changes.
- Residue from the hot water/chlorine that's not neutralized.
- Something in the gravel
- It could also be the amount of salt you're using to treat them.
One or several of those are causing your betta's fins to decay like that.
Either way, you need do some ammonia tests on your ZM houses to see what the levels are.
- BTW, what kind of sponge are you using to clean it with?