Pleco has strange circle

[Alcon]

I noticed my pleco had a strange circle on his side. He is about 18-20 years old and I've never noticed anything like this before.

Any ideas what it might be and how I cam help?

 

[Colin_T]

Looks like a small sore with fungus and excess mucous. The entire fish appears to be covered in excess mucous too. This appears as a creamy white film over part or all of the fish and is produced by the fish in response to something in the water irritating it.

Test the water for ammonia, nitrite, nitrate and pH.

Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.

Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. If there's no improvement after a few water changes, post more pictures.
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. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.

 

[Ichthys]

Reminds me of a limpet. Does it move?

 

[MaloK]

In addition to what Colin mentioned.

If you are able to touch the fish with minimal stress, You can clean the wound with mercurochrome or methylene blue. this will greatly prevent re-infection for a while and help give a good start healing.

As long as there's fungus on the wound, it will prevent it's cicatrisation.

 

[Alcon]

Thanks for some of the tips!