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I swap mine as soon as its not 0 zero tds but I rarely do water changes so most of what its for is top off. Possible that your town added more chlorine to the water or chloromines to combat algae growth?
They are super easy to hatch.
I used this
https://www.bulkreefsupply.com/brine-shrimp-hatchery-dish-brine-shrimp-direct-inc.html
Had excellent success with it.