20ppm isn't great but it wouldn't kill.I took my
skimmer off my tank to get ready for a new tank,it didn't come as expected.My nitrate rose to over 20ppm I did a 50% change last night.My animals are fine,I have a Coco worm and they don't like nitrate.If a Coco worm stresses it will lose one or more of its crowns,mine didn't.Nitrate at 20ppm didn't kill your inverts.What was your ammonia and
nitrite level at the time you added the snails and shrimp?
API Reef Master Kit doesn't have ammonia or nitrite what
test kit did you use?Is the water you add the same temp?What kind of water are you using?Have you tested the water before adding salt especially for copper?Tap water can contain copper deadly to inverts.Other than this I'm at a loss for answers.