GT possibly dying?

Hey all. I have a Green Terror thats about 2.5 inches in a 75g tank. The other members of the tank are a JD(2"), convict(2"), jag(1"), three texas(3/4"), 5 tiger barbs and a pleco. The GT, being the largest, had been the dominant fish since the start, they've been in there for about a week. All the fish swim around together and there doesn't appear to be much agression (at least since I replaced the jag with a smaller one).

Starting yesterday I noticed the GT's beautiful body not in the mix. I found him hiding in a rock cave and just hovering there, not moving even for feeding time. He seems to be breathing heavy. He stayed in that spot all of yesterday and this morning he is in a new spot doing the same thing. I can't see any signs of illness on him, he has become a little pale, but I see no parasites or anything. There is no physical damage that would hint towards a fight. From his physical appearance and my experience with GT's I have determined its a male, so I don't think he's trying to spawn.

Does anybody have any ideas? He went from the most active fish to not active at all! Any input is welcome, and thanks in advance! I don't wanna lose this lovely fish!

The water parameters are all good. The barbs have been in the tank for about 2 weeks, and all the cichlids for 1 week. The tank has been running for over a month, and I thought I did a pretty good job of cycling it. The only thing that has changed is when I replaced the jag with a smaller one, but they were both from my tanks so I don't think that would affect anything. Maybe one of my roomates did something to it. I was gonna do a water change, but how much should I do, 50%?

Too late he died. Thanks?