Jen Loves Bettas

While the KH you mention of 40 (& I just now see the update about 0…) doesn’t seem problematically low for the species you’re keeping, it’s easy enough to raise this alkalinity using common products. There are certainly alkalinity buffers available, but the thing I’ve used most over the years is baking soda (sodium bicarbonate). Typically, 1 tablespoon of sodium bicarbonate in 50 gallons of water will raise the KH by 2-3. So you can just go easy and measure there. My local tap water comes out between 20-40 dKH so if I’m not working with soft water fish like my wild bettas, I often add some bicarbonate to help the tank resist a pH crash and also to fuel the biological filter as Nora Loves Corys! mentions. Plants also appreciate this. As for UV filters, this might be helpful yes. I was able to purchase one on Amazon that I use in my quarantine tank. It’s sold as a green killing machine for nuking algae in the water column but they can also do a nice job of helping bring down levels of bacteria or other parasites that could cause issues. It’s like a small internal canister filter that you add into the tank. If you’re ready to potentially pay for a vet, it would seem a reasonable investment to try!