My half moon betta, Mongetsu seems to be a very sick fish lately. I rescued him from petco despite his cloudy and now have come here in fear of what might be his final days.

He is in a two gallon bowl with temperatures between 70-80 degrees. He has no filter but has an air stoneset in the middle. It isn't heated because the room hot enough these day that we fear he might over heat. He lives alone.
Right now he refuses to eat everything but brine shrimp. Though today he has refused everything offerd to him. He is fed three times a day sometimes he is fed a small snack.
Half his water is changed once a week with all of it changed once a month. we put in dechlorinator and ph balance though he was given stress coat today as he seems sick.
We havent tested his water yet.
His symptoms are:
one cloudy eye that he has had since we got him.
won't eat
just lays at the bottom of his bowl
his body seems a bit paler even though he is kept in the same light level everyday and the light is fairly bright.
He has always been lazy but not this lazy like I said, he just lays there unless the bubble get to close to him.
I noticed the cloudy eye as soon as I saw him, we got him so we could treat it. Other symptoms started early this morning.
We havent given him anything yet though his water look all kinds of nasty (as in gunk) so we rinsed everything and change all his water.