That is strange, everything looks so healthy in your tank. Do you drip acclimate them? Check that they eat at the LFS? What fish/inverts do you have in there now? Anything that's super aggressive to new fish?
I have been reading about hypo salinity for treatment of ich and it sounds like some may even recommended it for a QT period because it reduces their energy expenditure so they are able to eat more and "bulk up" before going into the tank. I've never QTd before but now that I have an "extra tank" I think I might for the new additions to the new tank. I know its probably a good practice but I'm impatient so we'll see if I keep it up.
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