11 dead cardinal tetras in last few hours

3 years ago I lost all of my tetras. The power went out and they went without oxygen for a little less than an hour and a half. Tetras can't breath from the top like your cory's can. I bet it was an oxygen issue. The power might have gone off and come back on. Some filters can self-start others need to be primed.
 
it happens sometimes when they do something to the water supply and at the same time frame you just happened to do a water change some fish are super sensitive than others, just bad timing on your part to do the water change during that time a little percentage of something that was in the water could have contributed to their death that wasn't treated or had a chemical in it. Just call the water treatment place and see what they were doing during that time that was not normal or out of norm... sometimes they're helpful giving you more info.

I don't believe it was a power-related issue with O2.
 
mayb there was a powercut while u were away? so then there would be lack of oxygen. And the power mite have just got bak be4 u came bak home so u didnt acknowledge it? cuz last yr there was a powercut at my house and 13 of my cardinals and neons died within an hour.
 
Thanks for all the responses.

I've been using the same food that I've had for the last few months. And all the fish pretty much eat the same thing - beef heart flakes, reg fish flakes and blood worms. Nothing really changed other than the filter cleaning a few days before. Water parameters are still great as of this morning and the remaining 2 cardinal tetras appear to be fine. All my discus and cory cats look great still. I'm the only one that really does anything with the tank, although I've asked everyone in the family if they know of anything that they might have done. I've thought about it all and have tried to come up with any scenarios - was there something on my hand when I tried scraping the brown algae off the glass? Was there a large spike or drop in temp during the water change that didn't kill the fish till later that night? Although that scenario, from my experience with my fish, specifically my discus, the discus would show more of an effect than my other fish.
 
As for a possible power outage, I've got so many electronics in my house that if the power went out, there would be some power failure sign. And specifically on that particular circuit, if power went out, the Fluvall 305 would have needed some help in being primed if it was out for a bit. It is possible I guess that it went out briefly, but don't see how it would have killed the tetras.

As far as something being done to the water quality prior to the change, my discus are the most sensitive when the water changes outside the norm. Any slight temp change or if ammonia, nitrites or nitrates show up at all in the slightest, their fins clamp down and they go dark.

The plastic plant they were mostly caught in is pretty dense...and it just seems like they all died at the same time, same place, except for 2 - but it's pretty common from watching my fish that 1 or 2 tetras get cut off from the group and are in a different part of the tank sometimes.

One other thought that popped in my head - I used the Seachem Safe powder during water changes. 1/4 tablespoon. I've seen my tetras swim over and eat some of the Safe as it drops into the water. I wonder if they all ate some of it earlier that morning and then it had an effect later that night on the group that ate it?

I'm going to place the tank on watch for a few weeks before trying to replace the tetras. A true murder mystery!
 
any live current in the tank?
 
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