Fresh dechlorinated water IMO/E is one of the best things for fish long term health.
Making regular water changes of at least 50% twice weekly in 6g and less aquariums with filters and 50% and 100% weekly on unfiltered tanks that have one Betta, more depending on stocking.
10g and larger at least 50% weekly depending on stocking, filtration and live plants use.
This is with uneaten food removed after feeding and not overfeeding
help oldfishlady ! yoshiko is not layed on the bottom anymore but instead he is just sat at the top of his tank next to the filter he hasent moved in a while! he is still full of coulor and is breathing fine.