common pleco acting very weird

Hi all I'm getting very concerned with how my common pleco (8") is currently acting. Usually he is very active usually stays in hiding during the day. However yesterday he came out and has been out ever since, when he swims he moves very slow and just sits there. His movement is so slow that when he sits you can't even see him breath he just moves his eyes at time to time. I even dropped some of his food in and he didn't even move when it landed on him. His coloring has not changed, for the past 3 weeks it has been a lighter black as apposed to the other pleco being a darker solid black. I could be stressing over nothing any thoughts would be greatly appreciated.
Tank is a 120g
Other fish in tank is
Another common (6")
Knife fish (12")

Well unfortunately he didn't make it. I lost him earlier today. When I got him he was in my quarantine tank for about a month and than was put in my 120g with another common and up till today he was in there for 6 weeks.i did take some pics of him just need to get them uploaded, maybe someone out there can help so I know if I should worry about my other plecos

Could be a gradual thing like parasites. Did you worm them in quarantine?

He died about an hr after being placed in quarantine while I ran to my lfs. A friend of mine who is a successful breeder of alot of differant breeds from down the street came over before I buried him and he looked it over and felt in his opinion that it was a male and had signs of fighting. He said in his opinion the other is a female and started attacking him trying to get back in her cave. Like he said it was only his opinion but we have seen it attacking it which just started a day or 2 ago. When I first put him in we were joking around saying they were showing breeding behavior but I'll never know now.