No point tagging me, I have about 15000 tags I haven't looked at
Probably a drug resistant bacterial infection caused by all the anti-biotics that get used in the US
I need a picture of the tank.
How often does the tank get a water change and how much water gets changed?
Do they gravel clean the substrate when they do a water change?
Do they dechlorinate the new water before adding it to the tank?
What sort of filter is on the tank?
How often does the filter get cleaned and how does it get cleaned?
Did they have carbon in the filter when they treated the tank?
If they did, that probably stopped the medications working.
Anti-biotics should never be used unless you know what the problem is because they cause drug resistant bacteria that kill things.
What other fishes are in the tank?
Is there anything sharp or pointy in the tank?
The fish might be rubbing that area on something and that is stopping it from healing.
What does the pleco get fed and how often does it get fed?
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Try doing the following.
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for 2 weeks. 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. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
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 them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
Add some salt, (see directions below).
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SALT
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, Bettas & gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria, fish, plants, shrimp or snails.
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.
When 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.
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FEEDING TIME
Feed the fish 3-4 times a day for a month to build up its immune system. Then reduce it to twice a day for a month and then once or twice a day after that. Do big regular water changes and gravel clean the substrate during this time to keep everything clean and free of diseases.
Make sure the pleco has some driftwood and algae to graze on. If there isn't any algae in the tank the fish could be suffering from nutritional deficiencies, which will weaken its immune system.
They need the driftwood to feed the good bacteria in their gut.
Increase the lighting times to 16 hours a day to encourage algae to grow on the glass an ornaments.
You can also feed the pleco on slices of cucumber, pumpkin, zucchini, spinach, marine algae, etc. Feed it some algae wafers and bottom feeding pellets too, as well as pieces of raw/ cooked prawn/ shrimp, bits of fish, and various other frozen fish foods.
*NB* Make sure anything you put in the tank is clean and free of chemicals, soaps, residue from hand sanitiser, etc.
Don't feed it potatoe or onion or any onion relatives (spring onion, shallots, garlic, leeks).
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If there's no improvement after a couple of weeks of salt, daily water changes and gravel cleans, and more food, there probably isn't much else to do except contact a fish vet and get them to take a swab of the area and see what is living in it.
If the vet says it has a bacterial infection, get them to culture the bacteria and find out what the best anti-biotic is to treat that specific bacteria. See if they can give the fish an injection of anti-biotics rather than treating the water.
If you have to treat for a bacterial infection, do it in a bare tank with no filter. Just have an air stone bubbling away, a heater, and a piece of pvc pipe for the pleco to hide in.
When treating the fish with anti-biotics, wipe the inside of the glass down and change all of the water before re-treating the tank. This gets rid of the dead bacteria and biofilm that will build up on the glass when using anti-biotics.
