I have used regular I/O in my reef tanks and have not had anything "bad" happen. My corals still grow and look just fine. The extra trace elements make a small difference in the end if you are doing regular water changes. I/O has those elements, just in a smaller amount and thus can be depleted faster. BUT if you do a water change and those elements are not depleted yet then you are fine. This would apply in a small tank where there is not alot of corals, or the corals are soft of lps
. When you start messing with sps
dominated tanks then it might be another story.
I prefer to use reef crystals BUT if I cant swing the extra $10 or so I use I/O. Switching salts should not be a huge deal like it is being made out to be.