Wondering why my goldfish died

We have a 1,000 gallon backyard pond which contained 20 goldfish and 1
catfish. It had become very green with algae and was getting worse.
We decided to clean it. We bought a 100 gallon container to hold the
fish while we cleaned the pond. Once the fish were in the holding
container, we emptied the pond and power washed the liner. We then
refilled the pond, added chlorine remover, let it sit a while, then
added the fish back. The fish were in the holding container overnight
for a total of about 20 hours. Once the fish were back in the pond,
we added Algae Blocker, a blue liquid which is supposed to block
sunlight and reduce algar. The next day, all the goldfish were dead.
The catfish survived. Does anyone know why? Any info will be
appreciated.

Thanks.