tetra tragedy

Question
Hi--

We have a 30 gallon well-planted tank that is about 4 months old. It's been running great--the plants are growing and healthy, the fish are active and their color is good. Everybody's getting along. We have 1 pearl gourami, 9 albino cory cats, 4 guppies (2 male/ 2 female), and 6 tetras. We've been doing weekly water changes-- about 30% of the water each time.

I did a water change yesterday and everybody looked happy this morning. Today, though, my boyfriend came home from work to find the water looking very cloudy and the fish either on the bottom  or at the top struggling to breath. Four tetras were already dead. He immediately started a water change to clear the cloudiness and it seemed to be helping, but we lost the other 2 tetras. Everyone else is now ok.

While doing the water change he noticed that some gravel at  the bottom of the tank had been dug up, exposing the bottom layer of substrate (it's Substrate Gold, kind of clay-like, and there's a small ball of it at the base of every rooted plant.) There was also a smear of the clay halfway up the tank wall--no idea how that got there. He also noticed a couple small guppy fry (not surprising--we had known both were pregnant, but don't want to breed them. I was thinking the pearl gourami would take care of any babies.)

I test the water twice a week and things are good--ammonia and nitrite are 0, nitrate is low, pH is fine, and phosphates are ok. We also de-clorinate the water before any water changes.

So, what happened? I'm assumming the tetras were the ones to die because they're the smallest, but I don't want it to happen again. Did the substrate gold cause the cloudy water? If so, why hasn't that happened before?--the cory cats find that stuff in their travels about once a week and make sure to tussle around it. I'm stumped. Any ideas you have would be great.

Thanks so much
Tomasina

Answer
Hello,

I am very sorry to hear about your fish. You must be devastated.

As the  fish died very suddenly and for no apparent reason, I would turn to the water being poisoned by something. I know someone who accidentally got a tiny amount of peroxide in their tank, and in 4 hours all his fish were dead. It is very easy to do.
If you accidentally got any sort of bleach, soap, oils, anything, that was probably the cause.
If you are absolutely certain that did not happen, my only other guess was  that fish waste created an anaerobic pocket in the Substrate Gold, so when this was released into the water by the Cories digging, the anaerobic pocket could have poisoned the fish. And yes, it was probably the cause of the cloudy water too.
Tetras are not quite as "hardy" as they are made out to be, and it is normal for them to be the ones to die first.

I hope all is back to normal soon and no more fish die.

Best wishes,
Kathryn
I hope no more of your fish