My betta is in 1.5-2 gal tank
soil and clay substrate.
Water parameters are 0, 0, 10. Temp are 32c (90F shouldn’t be a problem since it’s Thai temp)
No filter but 75% water change every 3 day
He doesn’t eat and sitting on top of substrate
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@Colin_T @Deanasue @PheonixKingZ @LostBear @Crispii
Oh Barry - I'm sorry.
That doesn't look good, does it? Those folded fins look like he is very unwell.
A 2-gallon tanks strikes me as quite small - are those small plants live, or is it just a background? What is the surface area of the tank - is there enough area for oxygen to diffuse into the water, and for your betta to air breathe? I can see you have some elodea (? - hard to tell) in there but there doesn't seem to be much else if the small stuff is background. How long has he been in this tank? Has he been in here for a while, or is this fairly new to him? (If he's just been moved in recently, there may be something about it which isn't suiting him and it's just catching up with him.)
I know bettas can live in quite small containers, but they don't necessarily thrive in them. Not just because of the problems with oxygen and water quality, but because although slow moving they are a curious fish that likes to explore its environment, and which like caves and little nooks to peep into - they also like plenty of live plants to provide cover and to stop them getting stressed. If he's got nothing to explore he will become depressed and lethargic. Bettas seem to be intelligent and personable fish - they need to be kept occupied mentally to be happy.
The very frequent large scale water changes may be distressing for him, too. You'd really be better giving him more room, a more interesting environment, and filtering and oxygenating his water. You mention the temperature not being a problem (not sure how Thai temp applies) but this seems excessively high to me - and higher temperatures also mean the water holds less oxygen.
Personally I wouldn't keep a betta in such a small tank - I'd reserve it for a couple of snails/shrimps.
I suggest you get him a bigger tank - 5 gallons at least, and plant it up, adding an aerator and filter to ensure he can breathe easily. He could be slowly suffocating. Just because a fish (or any creature) manages to hang onto life in a small space, it doesn't mean it is happy or healthy.