commandment 5 and 6 are not necessary.
5) Common Siamese fighting fish (Betta splendens) has been bred in water without tannins for over 50 years. They don't need it.
6) Bettas are Labyrinth fishes that can breath air from the atmosphere or take in oxygen through the water. They evolved to live in water with low oxygen levels.
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Is the fish still eating?
Did you remove the fish from the bowl and wash it all out?
If yes, did you use any cleaning products or a sponge from the sink?
Did you dechlorinate the new water before putting it in the bowl?
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You shouldn't do a complete water change unless the fish is sick or there is something in the water. A 50-75% water change done several times a week is normally sufficient to keep the water clean and helps reduce the stress associated with a complete water change.
Any new water should be free of chlorine/ chloramine before it's added to the tank otherwise the chlorine/ chloramine can poison the fish.
If you use a sponge to wipe the inside of the bowl, make sure it is free of soap and other chemicals. If it's a new sponge, wash it with warm water and a bit of washing up liquid, then rinse really well to make sure there is no soap/ detergent left on/ in it. Use that sponge specifically for the fish and not for anything else.
If the temperature has been fine for the last few months it is not the issue.