Instant Ocean is perfectly fine to use. This is what Randy Holmes-Farley, author of countless reef chemisty articles, has to say about his salt choice.
"Here's my rational for using Instant Ocean:
I do not think there is a "best" salt mix. Nearly all of them will work fine as long as you know their pros and cons.
I don't want excessive borate, which leaves out Seachem.
I don't want vitamins or anything else organic in my mix (because I doubt their utility, they degrade with time to who knows what, bacteria may thrive on them as I store new salt water for a substantial period, they are totally undescribed with respect to amounts or identity, they are not naturally present in natural seawater at appreciable levels, and because I've occasionally had them mess with my skimmer), so that tosses out some like Reef Crystals, hW Marinemix Plus BioElements, Kent, Coralife, and Nutri-SeaWater.
I don't want excessive calcium (long term use of limewater as I use drives up calcium, so I do not want it starting high), so that tosses out a bunch, such as Kent, Seachem, Coralife and Oceanic.
There are certain companies that I will not support due to their misleading claims and/or product lines. That tosses out a few which I won't detail here since it is my personal thought as opposed to a specific issue with their salt mix.
I won't use certain lines of natural seawater due to excessive metals in it.
That only leaves a few to choose from, such as Instant Ocean and Tropic Marin Pro. The remaining ones might all be fine for me, but IO is lower in cost, especially if you get it when it goes on sale (which it frequently does). It also has a very long track record of success in many aquaria with relatively few concerning issues of bad batches."
My personal choice is regular Red Sea salt. The Red Sea Coral Pro version is a little too high in calcium and too low in alk and messed with my dosing scheme. The alk level in Instant Ocean is a little too high for my liking.
