It depends on what type of corals you want to have, LPS, SPS, softies, etc. all have different needs. IO is good for just fish but your corals will probably suffer somewhat and have a slow growth rate if they grow at all if you don't dose. Your calcium and alkalinity are very important to the corals lives. I am finding this out as I learn this hobby and if you switch to a reef salt like Reef Crystals, a product of Instant Ocean and comparable in price, slightly higher, about $5.00, from Fosters and Smith. You may still have to dose alkalinity however. From what I understand, the alkalinity elevates the pH, which buffers your calcium which maintains its proper level. I am just learning this slowly but there are a lot of people who know more than I.