WC on the big tank get bad all the time

[HungDang]

I don't know what the hell is going on but whenever I do WC in my 1000 galon tank/pond I lost some fish to what I believe is Chlorine poisoning. This is ridiculous because when I do WC for my smaller (55, 10, 5) tanks it never happen. I use prime for water conditioner and treat the water right before, during and after putting the new water in!

 

[DammitKhoa]

ouch. i hope you're not losing some expensive/larger fish. it shouldn't be chlorine if you're using prime. do some water tests and you'll know if it is chlorine.

is the pond cycled?
how much water are you changing?
what is temperature of the new water coming in?

 

[HungDang]

its always get the big fish first, some time I can save them, sometime not. the pond is almost 10 months old, I usually change about 20% water and the temp of the new water is about the same as the old water.

 

[fook]

What kind of fish?

 

[HungDang]

koi and gold fish!

 

[Gill Blue]

is your water surface disturbed? I'd never had this problem until I went with a fully planted tank with no surface breakage.
I'd do a water change in the evening and I'd wake up to all my fish gasping for air at the top. moving the powerhead flow so it broke the surface of the water took care of it.
large fish need more O2, so I can see where something like that might kill them starting with the biggest.