I used to rescue bettas in NYC (the worst of the worst about to be flushed, as well had some laws changes). (the image from betta rescue is from fish taken from a NYC deli - they would sell then in in little bowls in freezing cold temps (doors open). I told them I would turn them in (health code issue) or let me have them. I later got this activity stopped.
If they come from overseas they are already stressed and over 50% die on the way. They are sorted out on tables, packed with meds (that blue water) in small plastic bags and shipped. I used to go in to the local pet store with an (P) and take as many as I could as most were flushed (not even put down before being in the sewage system which is NOT an easy death).
I kept them from a child back in the 60's and did massive research before ever buying any (and a background in pre oceanography and other sciences). Just as one would (hopefully) not bring a baby home without learning all about their care - ditto for any other living creature including fish which are as complicated a living organism as any, and they need specific environments and care.
......................................
You don't say where you bought him but if at a typical store where they are kept in small cold bowls, they are usually already sick. All sorts of diseases (gram negative bacteria and fungus enter at the delicate tail ends - a single layer thin). NO fish should have such constant "water changes" or disturbed OSMOTIC systems. I highly disagree with the 75% water change. Imagine you yourself in a room where each day your air or what ever was quickly and vastly disturbed.
.........................................
Bettas come from huge river systems, but slow black water systems. They do not ever do well in small set ups UNLESS expertly designed. Still they like any creature like to roam about.
I eventually figure out a way (much research from older books and a great deal of research from science) to treat the dreaded Columnaris (and this was when most were at an end stage - rescued from fish stores and alas at this stage many could not make it). These bacteria and fungus have already deeply invaded the entire organism.
https://search.yahoo.com/yhs/search...tail&hspart=mozilla&hsimp=yhs-100&type=newtab
Usually there is more then one “event” occurring. (Fungus, bacteria and possibly parasites)
..........................................
Someone mentions 75% water changes with de-chorinated water. Sorry but I highly disagree. He needs a sick tank set up. Plants (in this case fake) for security - all fish need this security. He needs a heater and a low level old fashioned box filter with floss (if you treat with meds). No charcoal as it will absorb meds. No meds and salt water cure charcoal aok.
The water during changes should be heated, the exact correct PH (and other parameters) and slowly drain some from the bottom and dribble (slowly) treated water from the top). Not much but x amount each day (or when mine were being treated more then once a day). The box filter helped a lot to reduce toxins and create less stress.
Again, fish are OSMOTIC.
They are living things and Osmotic at that. They have specific requirements and any shift in PH (water RO and other chemistry) creates great stress on their immune systems. Then those ever present bacteria, fungus and possibly parasites do what they do with them and us as well.
https://www.angelsplus.com/ArticleOsmosis.htm
My research is long stored but a slight PH shift of even .25 (and other vital water chemistry) can cause stress and disease.
...........................................................
I used to treat fin rot and other diseases with the old fashioned salt water cure (one must know how to do this as the amount is gradually increased over time then reduced over time - it takes patience) and sometimes meds. Columnaris needed specific meds in the food as it was systemic by then.
With severe fin rot (gram negative bacteria) (and some were rotted up to the main body so on their way out) I would put the fish on a wet slant board (excise the rotted tissue) treat with meds and back in the salt water tank. (don't do this, lol).
But at this stage that bacteria has invaded the arteries and eventually will other tissues then their organs. I fed live foods - later when they recovered a tad (like us when sick they don’t need to eat much), low light and fake plants to provide security and comfort (stress produces adrenal/cortisol hormones which can kill).
.............................................................
Complete Guide to Betta Fish Diseases and Treatment (with pictures)
https://nippyfish.net/complete-guide-to-betta-fish-diseases-and-treatment-with-pictures/
Breeder and great source of information (great and charts of meds so on)
http://www.bettatalk.com/betta_diseases.htm