Whenever you mix up salt with water I believe that you should heat the water first before adding salt and a powerhead to mix the salt. You should always test a new bucket or bag of salt for Calcium, Alk/Dkh and Magnesium. It is not uncommon for these measurements to change from bucket to bucket although they should be close or else you should think about changing salt mixes. It is also not uncommon for salt to be a bit low in magnesium. The reason I have been told when I asked was that it takes a whole lot of magnesium suppliment and it really comes down to $ to suppliment. Once the salt is added to the water and has a chance to mix for a couple of hours you should test for salinity (or SG as most of us do), PH and temperature. It may be necessary to add a buffer or other means of adjusting PH if it is not the same as your tank water. I disagree with Bev on the amount of time you can leave salt water mixing and airating but that is another discussion. I have from time to time added Magnesium to my newly mixed up salt water if I need to to bring that batch of water up to an acceptable reading. If it is your tank that is low on a particular suppliment such as Mg, Ca etc. you should mix it up with tank water and add it preferable to your sump but if you are sumpless then obviously directly to the tank.
I would hazard a guess that in your situation you may have had some wild PH swings that effected your livestock or something else that caused the oxygen in the water to be severly depleted causing stress to your fish. Give us your tank parimeters and I am sure everyone will try and help you figure out what happened.
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