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I had 15 beautiful koi, all about 5 years old. They were very healthy, but the other day I put too much Tetra algae control in their outdoor pond, and now 2-3 of my koi are dying a day.

If I put them in another pond I’m scared I will shock them. i only have five left; they seem to be ok, but they may be a little sick. One is just trying to breathe and swimming on its side trying to get air from the pump.

Is there something I can do to save them all? I put the sick one in a pond by himself just to see if he would get better. I didn’t do anything special to the water. I know I should have let the water sit a while, but I had to do something. I did put a small pump in the other pond. I’m sure I will wake up and find more dead koi. Is there any way to help them?

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First, increase aeration to maximize the concentration of oxygen in the water.

Stop feeding your fish to lower the waste production.

Clean your filter and remove waste from the bottom of your pond to lower the production of toxic gasses.

Do a large water change, up to 50% of the water, and remember to use dechlorinator (pond safe) or similar products before you start to fill your pond again.

One needs to overdose the algae control massively for it to be toxic to fish, so I think your die-off is a result of the algae decomposing and producing toxic gasses; remove the waste and your fish will recover.

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  • Waste ( dead plant material) removal is important . A third party put algecide in an aquarium I maintained and the fish were okay except the ones ( koi . goldfish , etc) that ate plant matter ( they died). Jul 19 '19 at 15:03

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