Dying fish

Question
I have a 5300 gallon backyard koi/shebunkin/goldfish pond.  I have a variety of plants.  I live in Northern Iowa and I keep my fish in the pond through the winter.  My pond has been doing very well and my fish have been thriving.  This spring, I have a very "green" pond.  I went to a local store and purchased an Algeacide.  I was nervous about treating my pond but the clerk said that it would be safe for my fish and plants.  Well, this is day 3 and I have lost 10 fish of 62.  Koi are taking the biggest hit.  I feel terrible but need to know whay I can do to get through this with the remaining fish.  I've been adding fresh water daily.  Any positive comments will help.  I feel like a really bad "fish mom".  
Thanks for your help.
Jody

Answer
Dear Jody,

Sorry to hear this.
The Algaecide can be the reason for this.
Although it is not necessary the reason ad hoc, however as the algae will die the oxygen levels will change.
Best way now is to change water as soon as possible for example with approximate. 25 percent.
Repeat this again within A few days.
Aerate the pond if possible.

Good luck!
Warm regards,
Raf