I have a 7 inch Red Tail Catfish that up until I did my last water change this afternoon was perfectly healthy... I have a 100 gallon tank and did a 40% water change due to some high nitrites 2 days ago. The levels were almost back down to zero for nitrites, zero for nitrates and .25 ppm for ammonia before the water change. I did lose an exodon and albino red tail shark when my nitrites spiked two days ago (believe it had something to do with adding a submergible rock background from universal rocks.com). Anyways, got that under control and like I mentioned, right after I did my water change and added API tap water treatment and nite-out II colony I noticed my RTC had his mouth open like a basking shark collecting plankton. Really weird. His breathing was erratic, like he would skip breaths and then open and close his mouth really hard and then stay stuck with it open. He started losing control of his body and was being twirled in circles by my wavemakers but then would get him self right side up and try to get to the bottom. He was never gasping for air at the top of the tank and I also want to add that my other fish are behaving completely normal. All water levels are normal, except for ammonia levels being at about .10 ppm. I was able to grab him by hand and held him in front of the wave maker's current hoping that the oxygen from the airline i placed behind it and the fresh water being circulated would maybe wake him up. I didn't know what else to do. I did this for a minute or two until he shook free from my hand. It has been a couple hours and after being completely vertical, to just being taken by the current, he has finally settled down between two rocks. I think he wedged himself in between them as to not be blown away because he doesn't seem to have the strength to control where he goes. Needless to say, he is right side up and breathing a little less labored. His mouth is currently not stuck open like it was a few minutes ago, but I have no idea what is going on with him. I have very fine sand substrate, is it possible that when i was filling up the tank a gas pocket stuck within the substrate got him or maybe he swallowed sand. Any advice or experience with this issue would most certainly be appreciated. Sorry for the long post, I just wanted to make sure I got everything down. Thanks