wow, 7 pages dedicated to this unfortunate occurance. Well, I haven't read every reply, but read a couple pages, and well...something has been happening to me too regarding IO. It seems that nowadays every time I mix up new salt to do water changes (~8gallons) and airate it for 24hrs, I get that chalky white film on the sides and bottom of my bucket too.
The last time I did a water change(less than 2weeks ago), the container I used to mix the water in was extremely filmy white on the bottom, so I added everything in the container except the bottom layer to the tank. The next day, my Tomato clown was dead. This is not the first time that this chalky white stuff has appeared after mixing. A couple of times before this, I've added the water to the tank, it clouded up, then a couple hours later, it was clear again.
Not sure if this had anything to do with my tomato clown dying, but my other fish survived (scooter, firefish, sixline, and gramma).
I have a 200g bucket of IO and this is the 2nd bucket I've used since I started this hobby. I have another brand new 200g IO bucket that I'm almost ready to open. Hope this one turns out different.
I do not check ALK on my mix. I will tonight when I mix up more saltwater (I'm curing LR in separate containers and have been doing many water changes).
The only things I test for when making a new batch of salt water is TEMP and Salinity. I would have assumed everything else to be ok. hmmm....
(I took pics of the filmy white stuff on my powerhead and container the day my clown died and can post later tonight)
just thought I'd add my experience.