There could be something in the water that is irritating the fish, or it might have a protozoan or fungal infection.
Fungus on fish is normally white and fluffy. I can't tell from the picture whether the white patch is fluffy and sticks up away from the body.
A protozoan infection can appear as a white, grey or cream patch or patches over the body.
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You can try doing a 75% water change and gravel clean the substrate each day for a week. See if that helps.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. If you use a power filter, wash filter materials in a bucket of tank water.
If there is no improvement after a couple of big water changes then look for a broad spectrum fish medication that treats fungus, bacteria and protozoan infections. Make sure it is suitable for scaleless fish (catfish, loaches, eels). If there are no instructions for use with scaleless fish, then use it at half strength.
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Wipe the inside of the glass down, do a 75% water change and complete gravel clean. And clean the filter before treating. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.
