fish died in large tank...WHY????

[terrors r us]

Hi there....I had some 3 inch red devils in a 750 gal tank and found them all dead...
This is the set up....a sump system that I just took offline for upgrading and a large
pond type canister filter that has back wash capability.

So I decided to do a back wash and accidently took out 75% of the water(you would thing that this would not be an issue) I also turned off the sump for upgrading and did not turn it back on....

I have done large back wash water changes before but I always turned the sump back on and never had an issue..

I slowly filled the tank back up over the next 2 days (I added prime and the temp. was good)

The only thing is that by keeping the water kevel lower.,....the water flow was very high
and poured into the tank creating mini water tornados....

After the water change I started up the large canister filter but not the sump....
Everything was good for three days and then on the morning of the fourth day I
found the fish dead...

The water was cloudy and there were little bubbles all over the acrylic(glass)

Very sad...I would like to know if I saturated the tank with oxygen or did I starve it
from oxygen???What are the bubbles about....even a week later bubbles kept coming
up thru the gravel......

Did the high water flow dropping into the tank kill the bacteria in the gravel???
By doing such a massive water change, did I kill the bacteria in the filter?????

This is so fustrading and puzzling....any ideas????????

 

[carsona246]

Why do you think this is an oxygen issue? if the canister filter is running heavily and there's surface tension there shouldn't be an oxygen issue unless I misread something. Did you cycle the fish tank with the sump? If so, the problem may have been that because you turned the sump off the bacteria from the sump was cut off from the tank, but you would think with 750 gallons a few 3" fish would be fine for 4 days. How many fish did you actually have in the tank?

 

[terrors r us]

the reason I say too much oxygen is , while the tank had a lower water level for two days the water pouring in the the tank made mini tornados which I thing saturated the gravel bed with too much air and killed the good bacteria and since the filter was just washed then is did not have enough good bacteria to keep the fish alive....this would explain the cloudy water...but what about all the bubbles all over the glass?????
When I tested for ammonia and nitrates...there were none....

 

[Dairy]

I'm not an expert, just an idea... Maybe the tiny tornados stirred up the gravel, releasing gunk and low oxygen underneath... Which were too much for the bio filters to handle. I think this happens with the natural ecosystems too causing massive fish kill. Just an idea...

 

[knifegill]

I slowly filled the tank back up over the next 2 days

After the water change I started up the large canister filter

Unless I'm missing something, you left your canister filter off for two or three days. That means you sent all the rotting bacteria and decomposition into the tank when you turned it back on. That's an easy way to kill fish. Filters should rarely be off for more than an hour or so.

 

[Spiritofthesoul]

3" in a 750g tank.

Did they find food?

 

[terrors r us]

Just thought I would update....there were 10 fish in the tank between 3 to 4 inches....
the canister filter was never off except during the filter backwash which takes about 15 minutes....fish were fed every day and ate like pigs...like usual.....fish were in tank for
3 months......the sump was turned off and drained and left off just before the water change...

I think the idea about the mini tornados which brought bubbles all the way down to
the gravel(for two days), might of saturated the gravel and disturbed the bacteria chain.......

Last time I did this I turned the sump back on right away and never had a problem....
I should of never turned the sump off and should of added instant bacteria from a bottle....

 

[smitty03281964]

Well you can not over saturate the tank with oxygen.I do not know what is in your tap water but maybe you did not have enough prime.

 

[carsona246]

I don't think disturbing the gravel would kill your bacteria, besides most of the bacteria would be living in your canister filter. Maybe if the water movement was that intense it just stressed the fish out too much. I don't think this is an oxygen issue

 

[RD.]

Sorry to hear this, I now those were some nice fish that didn't come cheaply.

How long was the sump turned off for?