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IO and Reef Crystals are made by the same company. The difference between the two is that Reef Crystals has a Higher Calcium level in properly mixed water, and i believe additional trace elements for corals and Inverts. IO is better for FO or FOWLR tanks, IMO.
I also use IO, but I'm a newbee. I have had no problems with it (except for caking). I buy it and dump it in a bucket andwhen I go to use it it's caked up. Oh well still disolves just fine.
Adding a small mesh bag of rice will keep the moisture from the salt and probably prevent the caking. I read once when the humidity gets to it and it cakes like that, the Mg has precipitated out to some extent, have no idea whether it is true or not.
justmy2cents i buy it in the bucket it is cheaper in the long run and the lid seals and keeps it from caking and it takes me a while to go through the bucket i used to buy it by the bag and it would cake up like what your saying which made it harder for me to keep my measurments consistant