It really depends on the corals. All corals that have skeletons will use the calcium, alklainity, and magnesium out of your salt mix to grow. Corals with and without skeletons will use a bunch of other elements out of the salt mix. How much they use depend on the type of coral, and like you mention, how many you have.
I would say that for the very large majority of folks, the only supplementation they will need (if any at all) would be calcium/alk
/magnesium. And this is easily done with any of the "2-part solutions" that are out there on the market. Or you can save $$ and do-it-yourself with kits like those sold on bulkreefsupply.com
You're doing a 30% change every other week, which is a lot... but good! Depending on how many corals you have, you *may* not ever need supplementation. Only way to know for sure is to test your calcium and alkalinity levels on a weekly basis. You want your calcium between 380 and 420 (in my opinion) and your alkalinity levels between 8 and 12 dKh
. If your levels drop below those targets before a water change rolls around, then water changes aren't probably going to keep up for you.
Your IO salt will be a little low on calcium, so you *may* need to supplement it right from the box/bucket/bag to get those levels. Instant Ocean's "Reef Crystals" has higher calcium and might work better for a reef. But again... only way to know for sure is to test.