[Taras1]

What's your SG/Saliinity? Instant Ocean usually mixes (according to the recipe) to about 1.021 which is a little low. That would then dilute out the Ca as well. Most "reef" mixes, like Reef Crystals, have additional Ca, but you also have to mix it a little stronger. Most people run their tanks at 1024-1025. Still it's not a bad idea to add Ca in a 2 part mix (a bit expensive but convenient) to get it up to 400-450.