Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Look up Lymphocystis.
If the spots have been there this long without affecting the fish perhaps they are a relatively benign growth.
If it is something that spreads, perhaps you want to isolate this tank, or remove the fish. Try a fish trap. Easy to make one with a plastic bottle (water bottle is about right).
If you are OK with isolating this tank, allowing the fish to live out their lives, then get duplicate tools for your other tanks, and keep separate tools just for this tank. Or else sterilize the tools between tanks, every time. Wash hands well, and always deal with this tank last, after finishing the healthy tanks. Do not share plants, filter media or anything else that cannot be sterilized. If you want to move driftwood or rocks from the affected tank to a healthy tank, then sterilize them.
Mardel is the name of a company. They make many medicines. What did you treat with? Active ingredient?