Sick Sailfin pleco, with large white sores

Hey guys,

I have a sailfin pleco around 3 years old, and in the last month he got some white patches on his skin. It looked like the skin had eroded to a white patch behind both pelvic fins (as shown in picture). He had this issue last month and I treated with API general cure and it seemed to help him clear it up. I had treated him twice consecutively for a total of eight days. Now a month later the same white patches with black speckles came back. I don't know what this could be and if I even treated him correctly from the start, or if it healing was a fluke. He has also acted out of character by darting and acting restless. This may be unrelated, but he also has shiny orange speckles on his belly, and the tip of the underside of his tail. I also noticed his fins look grayyed and are startung to fray.

Water parameter:
nitrite 0
nitrate 20
PH 7.5
KH 180
GH 0
Temp 79

Weekly 25% water changes
75 gal tank with no tank mates
He's been in this tank alone for almost a year


I would love any suggestions as I have treated previously and the problem has re emerged. Thank you in advance!!

Perhaps its something like anchorworm and he's scratched the trailing worm fibre off?

Perhaps. We did use API general cure again and are waiting to see results.