Salt is very good advice and it won't hurt a pleco, shrimp, snails or plants.
To cure the fungus I'd really go with salt, though I agree with you it won't cure stunted growth
You are using ±10 grams per gallon. It is way too much to prevent brown blood disease and for softwater SA fish but not enough for a therapeutic level against flukes. It won't do anything but stress the pleco if you are trying it for a bacterial infection or a tumor (which can not be cured by a product but needs to be surgically removed by a specialized veterinarian). Fungus (Saprolegnia) is not a disease that needs to be treated rather as a symptom of poor maintenance and weakened fish, which can and should not be fixed by salt either.
I still think it's bacterial not fungal but I don't know what antibiotic to suggest...Maracin 1 + 2 hit lots of stuff (at the same time)...but your poor fish looks in pretty dire straights. Stop cleaning the filters, you're just removing beneficial bacteria at most. Change water if you need to do something.
Do you have a recent ammonia, nitrite & nitrate test? Salt doesn't treat bacterial infections & will only irritate your pleco's already sore looking tissues.
9 years is fairly old for many fish. & as they age they are more prone to infections of any kind...& like others have said your tank is kind of small...that can add to their stress & reduce their resistance to diseases. I'm sorry I can't give better advice, I wish I could.
Look for a broad spectrum antibiotic. It will be more likely to work if one is unsure of the nature of the infection. Often a bacterial infection will also develop a fungal component as well. This is what I have used for years (it used to be called MarOxy) https://fritzaquatics.com/products/mardel-maracyn-oxy
This paper from the University of Florida will help you understand bacterial infections and the various antibiotics used for treating them. Use of Antibiotics in Ornamental Fish Aquaculture https://edis.ifas.ufl.edu/fa084
Bear in mind that it is the aquaculture and not the aquarium industry that drives a most of the research we ultimately apply in our tanks.
You are using ±10 grams per gallon. It is way too much to prevent brown blood disease and for softwater SA fish but not enough for a therapeutic level against flukes. It won't do anything but stress the pleco if you are trying it for a bacterial infection or a tumor (which can not be cured by a product but needs to be surgically removed by a specialized veterinarian). Fungus (Saprolegnia) is not a disease that needs to be treated rather as a symptom of poor maintenance and weakened fish, which can and should not be fixed by salt either.