reef crystal have higher levels of stuff that depletes faster and things that are more important, like calcium and magnesium. use the crystals if you dont mind spending a little more $ and there available to you.
IO is intended for fish only tanks, and reefcrystals are designed for coral reefs, with higher levels of calcium and other trace elements, I use reef crystals and have no complaints.
I made the mistake of using Instant Ocean and my calcium level took a dive. I had to add a calcium supplement untill the bag was gone then switched back to Coralife.
