Fish dying for no apparent reason, what gives?

[cdawson]

I find it hard to believe that any of the dead fish have no marks at all on them. Even a fish that died of CO2 poisoning would have marks on it of some kind, be it white patches or red gills. Your fish would show some form of CO2 related stress before succumbing to it.

 

[benedictj]

Another possibility here, what about one of the gouramis? Is the remaining one aggressive? What type of gouramis are/were they? I only ask because I have seen first hand what a grumpy gold gourami can do to a tank full of neons and guppies. It is hellish.

Also, I noticed you originally posted nitrite and nitrate levels but no ammonia. What's the ammonia? Zero I hope.

One more thing, can you post an am and pm pH reading? KH? And your Co2 levels am and pm? Without the raw data, this thread could go on endlessly without getting you any answers.

 

[thebach]

I have one gourami left, I'm about ready to give up, I would be happy if I could just figure out if it's c02 or oxygen related, but with an airstone going at night and no c02 injection at night it is starting to look like I have a little serial killer in my tank. He needs to be found and punished.