I just bought a small pack of it about 1 month ago as I did not want to open up my large bags with the humidity around here, so I bought the 10 gal pack. What I found out is:
When I started a new tank (10 gal) I initially bought a 10 gal box of IO salt. It was loose in a plastic bag and you could move bag around and the grains all separated, just like sugar. Fine, I had no problems with that box of salt. The plastic bag it was in had no printing of any kind on it, just a plain plastic sealed bag. All that salt dissolved in a few minutes without a problem and left NO residue in my mixing container.
Then I needed another small box for another 10 gal batch, and being too lazy to open up my big bag or bust into the big bucket, I picked up another 10 gal box at another store locally. The cardboad on the box was much heavier, and the bag was over printed with Instant Ocean all over it, and this bag was a vacuum packed bag, that felt like a brick of wet sand. My old packs of salt had batch or lot numbers on the box or bags, this box has no such numbers anywhere to be found.
It did not flow through the funnel like it usually does, and it had a damp appearance to it, but it did not have a leak in the bag as it was not clumped up, and when I cut into the bag you could hear the vacuum letting go....So it was packaged like that. This is the salt that leaves a residue that looks like fine beach sand pure white in color, and nothing dissolves it, not boiling water, H2SO4, or all the agitaition in the world. It almost makes you think that IO is adding white beach sand as a filler in the stuff. I liked IO up until I got this pack and it just sucks. It was lowest of any batch of saltwater in terms of calcium as well...Something has changed somewhere. Was this an old pack of salt, that I bought or a new pack....Any one have a clue? Usually the salt is merely in a sealed plastic bag and not vac packed like coffee is........I spent most of the day trying to call IO and see what they have to say about it, but the fellow is never around. I called him numerous times before on different issues with various things, and eventually he does return the call, but it can take weeks for that to happen, but I am not done with the salt issue as I want to now what that stuff is. Be a real %^*& if someone had black sand and Instant Ocean was adding a non soluable white filler material like sand.......Maybe they need to change their name to Instant Beach as it left a heap of residue behind.....