I went on a search for the answer to this exact question a year or so ago.
Advanced Aquarist's Online Magazine - Feature Article: Inland Reef Aquaria Salt Study, Part I
Advanced Aquarist's Online Magazine - Feature Article: Inland Reef Aquaria Salt Study Part II
And the above review doesn't tell you which is best, but tells you what salts perform closest to Natural Salt Water on a number of different tests and the totals were used to rank the top three as with kent and TM at the top and IO in third.
That said I have had issues with salt before and I'll explain. About a year and a half into the hobby I bought a new bag of Kent Salt and when mixing it I knoticed it was a little clodier than normal and took a while to clear up. After a water change my tank went down hill super fast and it took me many water changes with new salt to correct the issue.
My other experience has been with oceanic salt. Its not a bad salt, but I was constantly I mean constantly struggling to keep my alkalinity at acceptable levels.
So given my experiences I decided to try the other two that were recommended IO and TM. I first choose IO and while it worked well and was reasonablly priced I set out to use both for roughly 3 months which is aobut a bucket of salt each and determin which I saw beter results with.
I'm currently using Tropic Marine Pro Reef and will continue to use it. My corals have beter color and appear healthier, but it costs almost twice as much. 65 bucks for three months of salt though realy isn't that bad in my opinion if my tank stays healthy.
Should be noted though i'm running a CA and Kalk reactor so if you weren't you would probalby have issues with CA and Alk using IO.
Just my thoughts.
Brandon
