Can anyone help determine what killed fish after water change?

Fish Frenzy

Feeder Fish
Jul 16, 2020
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I have owned this aquarium for over 8 years and nothing like this has happened to the aquarium before. 10 African cichlids just died belly up on the bottom of the aquarium all within 24hrs of the 60% water change, with 2 survivors a central American cichlid Jack Dempsey. I usually change the water once a week, but it has been a week and a half. The power went off for 6 hours 2 days before the water change and the fish were healthy/ active, chasing and eating food normally before and after the water change.

I have done much larger routine water changes without any issues before.

How can I tell if the cause was from high nitrates? or if I changed the temperature of the aquarium too quickly(I think I may have added hotter water, which I have done before with no problems)? or even the pH?
It definitely had to have been shock from something right?

75 Gallon;
mixed African cichlids
2 canisters

Parameters (months prior similar, and after the fish died tested)
Ammonia: 0
Nitrite: 0
Nitrate: approx. 80ppm (due to uncleaned filters 1.5 months, which got cleaned after the fish died)
pH is usually 7.6-7.8 in the aquarium and from the tap 7.8

Thanks,
 
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tlindsey

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I have owned this aquarium for over 8 years and nothing like this has happened to the aquarium before. 10 African cichlids just died belly up on the bottom of the aquarium all within 24hrs of the 60% water change. I usually change the water once a week, but it has been a week and a half. The power went off for 6 hours 2 days before the water change and the fish were healthy/ active, chasing and eating food normally before and after the water change.

I have done much larger routine water changes without any issues before.

How can I tell if the cause was from high nitrates? or if I changed the temperature of the aquarium too quickly(I think I may have added hotter water, which I have done before with no problems)? or even the pH?
It definitely had to have been shock from something right?

75 Gallon;
mixed African cichlids
2 canisters

Parameters (months prior similar, and after the fish died tested)
Ammonia: 0
Nitrite: 0
Nitrate: approx. 80ppm (due to uncleaned filters 1.5 months, which got cleaned after the fish died)
pH is usually 7.6-7.8 in the aquarium and from the tap 7.8

Thanks,
Welcome aboard
Did you test your water parameters after the power outage? Also the filters not being cleaned may have caused the old tank syndrome and that with the warmer water change may have caused a drastic PH spike.
 

Fish Frenzy

Feeder Fish
Jul 16, 2020
3
3
3
52
Welcome aboard
Did you test your water parameters after the power outage? Also the filters not being cleaned may have caused the old tank syndrome and that with the warmer water change may have caused a drastic PH spike.
No, the tests I have done included a couple months prior with those results, and after the water change to check on the fish. In hindsight I should have checked the perimeters after the power outage as things could have changed. I thought leaving it a few days should have been enough to let things balance.

I will look into old tank syndrome, yeah I think you may be right about the PH spike.
 

Fish Frenzy

Feeder Fish
Jul 16, 2020
3
3
3
52
What’s your source water?
Well or municipal.
Could be a change in the source water, especially if your are on municipal water.
Power outage could have caused your biological bacteria to die off.
Municipal water source in a large city, thats what I originally thought about the biological bacteria letting it die off. But the parameters with ammonia and nitrite, as well as the fish seemed fine. Just ended up belly up in the morning, 24hrs after the water change. My second aquarium much larger, is completely fine so this is where I am wondering what happened.

Luckily two fish survived out of 12, a Jack Dempsey and a Starry Night cichlid.

I haven't added any new fish in the aquarium for over 1.5 years.
 
Last edited:

Hendre

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Jan 14, 2016
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Nitrates would not cause a sudden destruction of a tank like that. What did you test with? Strips or liquid test?
Warmer water is usually less stressful, lower chances of sudden gas embolism. Not adding dechlorinator would be my first thought, or sudden ammonia poisoning from chloramine releasing ammonia (could have been metabolised by the time you tested)
 

agent1207

Feeder Fish
MFK Member
Jan 17, 2020
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Toronto, Ontario, Canada.
Did you forget to add dechlorinator?
No I have not forgot the dechlorinator, as I have a set way of dosing the aquarium. Before and after adding water just in case. The dechlorinatior works, as my other tank is fine.

But the question is, what ever it was how did 2 fish survive? I am very glad they did.
 
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agent1207

Feeder Fish
MFK Member
Jan 17, 2020
30
14
8
Toronto, Ontario, Canada.
Nitrates would not cause a sudden destruction of a tank like that. What did you test with? Strips or liquid test?
Warmer water is usually less stressful, lower chances of sudden gas embolism. Not adding dechlorinator would be my first thought, or sudden ammonia poisoning from chloramine releasing ammonia (could have been metabolised by the time you tested)
I use the liquid test, takes like 5 to 10mins. I agree by the time I got around to testing things could have stabilized again.
 
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