What size pond ? Cu killed the algae and the plankton and the dye stopped any new or remaining plankton from getting the light needed to power photosynthesis and produce O2 plus the dead algae used up what little O2 that was left. Did the event kill the FH ? If so you probably had a complete O2 depletion as they can tolerate low O2.

Wait until the Cu and dye are gone from the system (a mth) and then try a few FH. If they do ok for a few days then you can stock other fish.