Wondering why my goldfish died

We have a pond in our backyard in Tennessee. We have had it for four
years. We had twenty goldfish and 1 catfish in it. It had become
very green with algae so we decided to clean it. We bought a
container to put our fish in while we cleaned it. We put the fish,
their current pond water, and the water hyacinth in the holding
container. We then emptied the pond, power washed it, and refilled
it. We added chlorine remover after we filled it. We added the fish
back to the water. Then, my husband added some algae blocker (blue in
color) to the water after the fish were re-introduced to the pond.
The next morning, all the goldfish were dead. We are wondering what
went wrong. Can anyone offer any suggestions?

Thanks.