All my cichlids died in 48 hours

Hey guys I have a 30 gallon tank with 6 african cichlids...well used to have 6 african cichlids. Anyways the tank was cycled and everything was great for a few weeks, then I did a water change and did a bit of vacuuming of the sand/cichlid gravel mix and withing 48 hours all my fish were dead. I tested the water twice with no signs of amonia, nitrate or nitrite,the PH and water temp were perfect.

My dad told me that all the fish were swimming at the top as if feeding or gasping for air. All my water tests were fine so I tested the tap water and that was good as well.

Any idea what could have killed my fish?

how did you cycle this tank ??
it is odd there are no nitrates( I have seen that in a planted tanks)
I suspect either. pocket of toxic gas or some foreign chemical may have made it's way into the tank.
 
The 0 nitrates looks 'fishy' to me...
A tank with 0 nitrates hasn't been cycled.

Wait, do you have city water? Or is it from a well?
Something is not right, you'll probably get a reading on something on your tap. My tap shows .5 ammonia(from chloramine) a trace of nitrates and a PH of 7.2.

I don't think your tests are accurate.
 
The one thing I want to know most is where did that bucket come from and was it rinsed before using?

Granted, I know nothing of poisonous gas bubbles or failed chemical tests... I use small to mid-gravel for substrates and though I have run pH and chem tests before, I was never really strict and rigid about it.

What I [d]DO[/d] know is noxious chemicals and how easily they can end up in your tank. Also, I can see how even the smallest amount of such chemicals could easily upset the environment so quickly since the tank is so new and there are so many cichlids in it.

Also, if the culprit is an antibacterial cleanser, wouldn't it destroy all or most of the bacteria in the tank, leaving no traceable nitrites/nitrates? I don't know how some chemical cleansers affect those levels in order to destroy bacteria and microbes... but is it plausible that this could be an answer to some of the questions raised here?

I dunno. I'm just thinking off the wall, here... just a theory and all...
 
something killing the bacteria should show everything in the water tests - ammonia (rising) and nitrites (a small stable amount) that can't be processed, and nitrates (also stable but higher than nitrites) from previous bacterial conversion.
 
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