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.

Thanks,

Neil

Thanks for your help on this guys, seriously I am really quite amazed at
the response I've had and the helpful advice I;ve been given.

I added some cycle to the tank and now 2 days later its pretty clear
again and the fish seem good, a few shrimp have returned to their duties
already. I'm wondering if I should do a small water change now what do
you think?

Neil