Probably a sore from rubbing on something sharp in the tank.
Wipe the inside of the glass with a clean fish sponge.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the lawn.
Do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If there is 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 for every 20 litres of tank water. Keep salt in water for 2 weeks and see how the fish goes.
The salt will not affect the beneficial filter bacteria or plants.
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.