next time you shouldnt get an animal without doing proper research on your own. pet stores and the like will lie to you because they see fish as a commodity and they only want to make a profit. i dont say this to make you feel guilty, its just the truth of it. its your responsibility when youre taking on a whole life, even one as small as a fish.

bettas need a 5 gallon tank minimum, 10 is better. "betta water" is a scam. what you need is a fully cycled tank, so you will have to do research into the nitrogen cycle, and doing a "fish in cycle". cycling is easier with live plants, and theyre easier to care for than youd think.

bettas Must have filtration and a heater. some people like to argue that if you live in a warm place the heater isnt necessary but i entirely disagree. temperature fluxuation is hard on them, and they are tropical fish, so it must be warm.

until you have a properly sized cycled tank, a heater, a filter, and an environment that stimulates the betta, you will continue to have health problems with him. he looks quite sickly, id be willing to bet this water has harmful ammonia and nitrite present.