You said no additives for water changes? Does that mean you do not use water conditioner? Tap water contains chemicals which Are harmful to a betta and any other aquatic pet for that matter. Long term exposure just is not good for them. I recommend getting a water condition to add to new water if you do not have one. Prime, as others have mentioned, is a good one.



Unless you live in the tropicals, it is probably too cold for him to thrive without a heater. Even a small one would help. 78-82 degrees Fahrenheit, or roughly 25-27 degree Celsius, is best for these fish. Bettas have much better immune systems when kept at a proper consistent temperature and also look much brighter and more colourful.



Also, it is best not to add hot water for water changes unless you allow it to cool it down to the same temperature as before. It is important to keep the water temperature stable as to not cause stress or shock. I know because the building I live in have automatic faucets in the bathroom and they run hot water, so I have to wait and acclimate my fish back slowly because the water is almost 100 degrees Fahrenheit straight out of the tap! Not good for fish at all.



As for your little guy's fins, I think fin biting or possibility the beginning of some fin rot on the tail. I agree with everyone else on the Aquarium salt. Good luck.