try reduce the water level shallow enough ... say if the fish is 3cm in height, put 4cm of water. that will help a little. 100% of water change plus a little of salt with the new fresh water. Remember aviod giving the fish any temperature shock durnig the changing water steps. Heavy aeration at one corner. stop feeding for 3 days. Don't worry, they can live without food for 7 days.

Try and see if there's any improvement. Normally, you'll notice the improvement in 2-4 days.

Good luck...

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Japanese Ranchu is another ball game