I agree with
@casportpony 's advice. If it was only the ducklings showing symptoms .... probably just shock from cold water.
Though if you're concerned about the toxicity of your water, and you're waiting on results from a water test, I'd ... probably also hold off on the pool. But for future reference, a de-chlorinator (the Fish Prime) would not help a toxicity issue. Because chlorine actually
sanitizes water. In fact,
in an emergency situation, adding chlorine bleach to water is recommended if you do not have access to bottled water.
But chlorine alone does not sanitize everything. A good water filter is more useful - though they also have limitations. I would suggest doing some research into what specific protocol your state uses for eliminating zebra mussels, and then researching if there's filters on the market that can filter those chemicals/contaminants.