one fish, two fish, dead fish... new fish???

Something is wrong with Quoc's fish tank. We have no idea what happened, but this morning when we woke up, all the fish were at the top of the tank, gasping for air. Quoc has done 4 water changes today and vacumed the tank several times. Each time, everything seems like it's back to normal, and then all of a sudden, they're back up at the top of the tank, sucking air. We thought that we had it all figured out before we left for our friend's house to watch the superbowl, but when we got home, there they were, all up at the top again. My thought is that there is something wrong with the filter and it's spewing out something that's spoiling the water. It's a long shot, but I can't think of anything else that Quoc hasn't already tried. Please pray for our poor fishies. We've lost one so far, and a second one looked like he was on his way to fishy heaven, but he made a speedy recovery once Quoc did a water change. I just hate the thought of all of them suffering because we've done something wrong with their tank. I just can't imagine what happened overnight...