Debbi's right, (wow, I've never seen so many users in agreement!) More water changes are better and clean water is the single most important thing you can do for a fish who's recovering from anything. If you can do every pther day, great. If not, do the best you can. Just make sure you syphon out uneaten food every day, water change or not.
You can also try pelets or bloodworms, or whatever other fish foods you have laying around the house- most plecos are not strictly vegitarian, and some even need a mostly protein diet. Look into this specific breed.