# Dead fish after 90% water change?



## markot (Aug 29, 2011)

I was moving my 20 liters aquarium yesterday to a new location across the room and to do that safely I took out about 90% of the water. I left the fish in the remaining water in the aquarium and refilled it a few minutes later.
This morning I found one of the Tetras dead and another one is swimming weird.
I did use water conditioner right away after refilling the tank.

Any idea why this happened? Aquarium was running over 6 months without any problems (apart from algae growth) so far.


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## Lieslq (Mar 13, 2012)

water coditioner should be added before you put the water in the tank, i think a 90% water change is a bit extreme for the fish to handle i normally never do a water change larger than 60%. dit you leave the water to reach room temp before you added it to your tank?


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## markot (Aug 29, 2011)

Lieslq said:


> water coditioner should be added before you put the water in the tank, i think a 90% water change is a bit extreme for the fish to handle i normally never do a water change larger than 60%. dit you leave the water to reach room temp before you added it to your tank?


Yes, the water was the same temperature as before +-1C. I guess I should have added water conditioner before as you mentioned.


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## navigator black (Jan 3, 2012)

It's how you manage the tank before a 90% change that determines if the change will go smoothly (now someone tells you). If the temperatures were right and you dechlorinated/dechloraminated, then you should not have had problems unless the tank was dirty.
Do you do regular 25% water changes weekly?
If not, organics will build up in the water, changing its chemistry. The fish will adapt to the stress of the dirty water. The pH will alter. If you top up without water changing, it will harden the water (minerals don't evaporate). Suddenly, you remove all the gunk, minerals, acids, etc, and add fresh, clean water.
Fish have complex 'osmotic' systems, to balance the amount of water in their bodies against the water outside. A radical water chemistry change alters the balance of minerals, etc in the water, and produces an enormous and often fatal stress on the fishes osmotic regulation system.
This is why it's bad to change water once a month, rather than weekly or bi-weekly (at most). Slow and steady keeps the balance. If I am moving a tank (or when I moved house), I did 25% then 40% then 50% in the 3 weeks preceeding the big move, and the fish came through swimmingly. 
I have killed a few tanks of fish by letting them go, and then deciding to clean them up suddenly. Now, I never top up - I change.


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## markot (Aug 29, 2011)

I was changing 25% of water every other week, I should have probably be doing it every week. Lesson learned.


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