Fish Gathering at Surface After Power Cut

Nick Park

I have a 5-foot 450 litre tank with assorted fish including a bunch of mbuna. It has two filters, an Eheim 2229 and a Fluval FX6.

Yesterday we got hit with the snow and the power was off for an extended time. I added warm water but many of the fish were looking the worse for wear, probably because of lack of oxygen. I drove through snow drifts to borrow a generator and hooked it up 10 hours into the outage. At that point I got the Eheim up and running (since it has a spray bar), two air stones and the heater.

The fish revived pretty quickly and everything looked OK.

The FX6 was off for 16 hours. I was worried that the bacteria would have died & turned poisonous, so I cleaned the filter out, washing all the sponges & media in tank water and putting new carbon & water polisher in it.

A couple of hours after turning the FX6 on again, the mbuna began gathering at the surface (a parrot fish and a couple of catfish seem unaffected). Since the FX6 seemed to have triggered the problem, I disconnected it and did a 50% water change. But the fish are still all at the surface - all at one corner of the tank.

Ammonia is 0, nitrites are 0 and nitrates are at 40 (so usual parameters). pH and water hardness are normal. Temperature is back to normal (28 centigrade) and water should be oxygenated ok as spray bar and air stones are running properly.

Any advice as to what is happening here? I'm really nervous that I'm going to lose all these fish.

Sleepy

Did you clean all filter media in regular tap water? You'd have decimated your bacteria colony if so, causing the tank to begin it's cycle again. Luckily there'll still be some bacteria in the gravel etc.

40 is rather high for nitrates imo.

I'd be doing daily 10-20% water changes for the next while!

Nick Park

The media in the FX6 was washed in tank water, but I think the bacteria colony at that stage was decimated anyway due to having no oxygen flow for 16 hours. At that point, the issue was probably more about the dead bacteria poisoning the tank rather than anything else.

(Btw this is why I always have two filters running on all my tanks. If one breaks down then the other one will contain enough bacteria to prevent a disaster. I also like to massively over filter my tanks).

I had thought thoroughly washing the media in the FX6 would remove any poisonous stuff, but I never thought about the outlet hose - so I suspect that's what was causing the fish such distress once the filter was hooked up again.

Anyway, another 50% water change and installing another powerhead with venturi to increase oxygenation seemed to revive all the fish and the tank is pretty much back to normal.

There was just one casualty, a 14-inch Black shark catfish couldn't handle the changed water parameters caused by the large life-saving water changes. When I was out yesterday, he jumped out of the tank despite there being a hefty cover on it. Found him dead on the floor.

Still, one fish lost as opposed to a complete fishmageddon wipe-out is a better than expected outcome.

cichlid child

Have you checked your Ph