Check the water quality for ammonia, nitrite, nitrate and pH.
Do a 75% water change and gravel clean the substrate every day for a week and see if it improves.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If there's no improvement after a couple of big water changes, add some salt.
Add 2 heaped tablespoons of rock salt, sea salt or swimming pool salt per 20 litres of water. Keep salt in the tank for 2 weeks and if there's no improvement after that, post some more pictures.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.