PH is 7.2, Ammonia, Nitrate, and nitrite all read 0 when I check last night. Temperature is 77. Only other tank mate is a fingerling bass. It's in a 55 gallon
Hes only about 6 inches long, I've had him going on 5 months now.
I found I wasnt shaking the nitrate test hard enough causing a false reading, it is now showing 10ppm.
I would suggest keeping a close eye on him and if it starts spreading maybe look into salt dips for a few days.
Even though the nitrates are low, increasing water changes for a couple of weeks may help as well. Sometimes extra water changes gives the immune system the boost to do whatever it needs to kick out the infection.
What are you feeding it beside zucchini? Most plecos are omnivores & a 6 inch fish will need a tablet or wafer or 2 almost daily. Undernourished fish are more prone to diseases.
I agree with Tiff, more water changes are always a good first step.
Maybe internal parasites since he looks thin around his head & back & it sounds like you feed enough. Has he always looked so thin? How much has it grown in 5 months?