All fish died last night

[adromero]

If there was a deep area of sand that was anoxic and was disturbed it could have released hydrogen sulfide gas, is this a possibility op?

I don’t remember stirring the sand bed but I do use chemi pure and nopox (3 ml daily). Would my phosphate level tell me if I’m overdosing

 

[Mjrenz]

I don’t remember stirring the sand bed but I do use chemi pure and nopox (3 ml daily). Would my phosphate level tell me if I’m overdosing

I haven't used chemipure or nopox before so I can't offer solid advice on them, I'm sure somebody else can though

 

[SaltyFoxfish]

I would test ammonia and nitrate, then retest PH. Just because a tank is cycled doesn't mean a spike can't happen and often a sudden die off can be seen by changes in ammonia and nitrate. Many chemicals will leave an ammonia trace after exposure and sudden changes in PH could indicate a lack of oxygen. Generally speaking, a sudden fish die off is a good time to not only run a full set of tests, but take a water sample to you LFS and have them confirm your results.

 

[rob safron]

Sucks. Aside for some toxin, chemical getting in the tank, I agree Oxygen depletion could be the issue. Think you mentioned some fish jumped. Many years ago I had a hang on back filter that lost siphon and I found all fish dead and a couple fish jumped. Corals we’re all fine.

 

[motortrendz]

Stray electricity? Maybe a powerhead or a heater leaving voltage o to the tank.... just a guess