Neil Woodman wrote:

Hi,

If wonder if any of you guys could help me. I did my usual 30% water
change on Saturday night, I got in on sunday to find all of my tetras
swimming near the surface gasping for air, the only thing I had done
differently was increase the CO2 slightly. Then I noticed some bleeding
heart tetras were dead, and then I noticed some shrimp going a bit
strange darting up to the surface and back down again. Suddenly all the
fish started to die in front of my eyes! There was obviously no oxygen
in the water so I added a powerhead to create some surface agitation and
did a 50% water change. The tank has now gone really cloudy and I've
lost most of my tetras and shrimp, there are only some ottos, a 4
tetras, a rabble of shrimp and one ram which is hard as nails left
alive. So this morning I rushed to my local aquatic shop. He asked me if
the water had gone cloudy after I explained what happened and it had, so
he then said it was a breakdown in the filter which has caused all the
bacteria to go into the water, which had in turn suffocated all the
fish, he recommended doing a big water change which I had already done
and recommended I use some filter aid to clump together the particles to
filter them out faster, I cant remember exactly what he said but it was
along those lines. Can you guys explain what might have happened and
why? This has never happened to me before and I've been keeping fish for
a long time, to suddenly see them dying in front of my eyes has been a
harrowing experience.

I use an external filter and pressurised CO2, I dont know if that's
anything to do with it, the tank is heavily planted.

I'm very sad about what has happened.

Do you have any idea what the CO2 level was or the pH? Is the change
water straight from the tap and do you allow the water to stand to allow
noxious gasses to dissipate? My tap water contains high amounts of CO2
etc. Why do you do large water changes?

A breakdown in the filter causes all the bacteria to go into the water?
I don't think so. The bacteria are already in the water. Did your filter
jam up? You didn't make this clear.

Nikki