What are the ammonia, nitrite, nitrate, pH readings and do you know the GH
and alkalinity /KH
Do 4 pwc
of 25%, that way the change in water parameters will be gradual to allow as little shock as possible to the fish. Do 2 back to back then wait a couple hours and do 2 more.
Check the water parameters again.
If not in safe zone, do 2 more back to back.
Yes, skip feeding for a day, and feed very, very lightly but AM and PM. This will allow the food to be eaten quickly and also not fall to the floor and be ignored. Less food, less waste eliminated, less ammonia, less food turning to ammonia.
What is the temp of the tank?
Add an additional air bubbler. Or reduce the water level a couple inches and get some good splash going to add more aeration.
Are the gills bright red or brownish. Brownish would indicate Nitrite poisoning.
Have you changed the filter pads recently? If so you could be experiencing a "mini" cycle, not having enough Beneficial Bacteria to proces waste effectively.
If you have Prime water conditioner you can use the maximum recommended dose for emergency treatment.