Salt premix problem

In my experience most of it dissolves anyway, but it couldn't hurt to swish it around some more with your hand just to make sure. I'd mix it the best you can, and then add the sand because the powerheads will mix everything up anyway.

First what salt mix are you using? some leave a sediment in the bottom and don't mix as well. I gather you are mixing it in the tank? remember to leave room for the sand as it will displace water when you add it. Turn off the powerheads and place a the sand in the bag in the water and gently slide the sand out of the bag, thereby creating as little dust storm as possible. Do not rinse the sand prior to adding as some suggest.....the micro dust will help in the denitrification process and all the dust will settle after a couple of days once each grain becomes coated with bacteria. Be patient it will settle but takes time. After a couple of days you can start up the powerheads again.....are you adding live rock? if so you should add it prior to the sand so that it sits directly on the bottom, less chance of it shifting later........

where is the rock right now? From your post I thought you had just mixed saltwater in a 90g tank.......without anything in it?

I just mixed some instant ocean in a bucket to cure my live rock, and there was some sediment in the bottom so I guess it is normal and does not hurt anything. I don't notice it in the tank.