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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.
I cant really say exactly why the colonies died as ive spiked my alk from 9 to 13 over 24 hours and didn't lose any acros. However if whatever waa causing this maybe a bacterial bloom(white cloudy tamk) that usually removes most of the oxygen in the water. Wrasses need more oxygen to survive...