Apologies in advance for a bit of a double post, but I thought I'd share a bit of info I received from Seachem that was news to me. Maybe it will help you avoid repeating my unfortunate experience and some good will come from my loss of some favorite fish.
Long story short, 24hrs after a water change I discovered a fish kill where I lost approximately 20% of my fish. I had always dosed excel at an increased level during water changes. I ended up contacting Seachem at the prompting of another member here on the forum. Seachem stated that excel acts as an O2 reducer as do water declorinators. The combination of water treatments, at the same time, likely combined to reduce the O2 levels enough to cause the deaths. They recomended waiting a day after a water change to dose excel.
Here's a link to the original thread if you're interested in the long story:
http://www.aquariacentral.com/forums/showthread.php?t=232193
Long story short, 24hrs after a water change I discovered a fish kill where I lost approximately 20% of my fish. I had always dosed excel at an increased level during water changes. I ended up contacting Seachem at the prompting of another member here on the forum. Seachem stated that excel acts as an O2 reducer as do water declorinators. The combination of water treatments, at the same time, likely combined to reduce the O2 levels enough to cause the deaths. They recomended waiting a day after a water change to dose excel.
Here's a link to the original thread if you're interested in the long story:
http://www.aquariacentral.com/forums/showthread.php?t=232193