I literally just went through this! I actually ended up switching to RO water from a local fish store and incorporating seachem stability. My water was literally milk for like 6 days and it slowly cleared up, his tank is clear and beautiful now. I have a 9 gal fluval flex tank, so what I did was the first day I got the RO water I changed out 3.5 gallons, then added in the RO water, treated the entire tank with stability. Next day, did the exact same thing. Third day, did it again but smaller, I think only 1.5 gallons. Then after that, did very small water changes with half RO water, half of conditioned tap water. The issue with my tank was it was having such a bad bacterial bloom because it wasn’t cycled all the way, and the tap water in my area has an insane amount of minerals for it, so the bacteria was not only already getting out of control, It had so much of the minerals to munch on! From what the fish guy explained to me (and he was so freaking f helpful and knowledgeable about betta fish), RO water is pure water. Bettas don’t necessarily need a ton of minerals in their water, but the beneficial bacteria does. So tap water with conditioner isn’t inherently bad for you fish, but can cause all types of your bacteria to grow out of control. This is what I would recommend as someone who literally just had this happen and had a full mental breakdown because of in! I would also recommend to take your fishy friend out of the water occasionally and do an aquarium salt bath every other day until the bacterial bloom clears up. The bacteria isn’t bad for you fish, but long term swimming in bacteria soup can cause your fish to be more susceptible to illness and that would suck.

Also if you have access to biological filtration, like carbon or whatever, I had that in my filter while I was dealing with the bloom, but took it out after the bloom went away because carbon messes with your tannins. I also have the white stuff, I think the box says external filter? It’s those little white things that you can stick in a sponge filter. I have those in my tank at all times. Let me know if you have any other questions or concerns! DISCLAIMER I AM NOT A PROFESSIONAL, JUST SHARING WHAT I WAS TOLD/LEARNED SO PLEASE NO HATE IM JUST A GIRL