calcium levels

this dosn't seem to be in an apropriate spot to post this, however i didn't know where else to put it. I have been considering corals lately, and have been getting more into inverts. I do not add calcium to my tank, but understand that corals can deplete your system of calcium. I tested my calcium levels today and they seem rather um.....perfect, and well a little too perfect imo for a tank that has not had calcium added to it in years. My test kit does have a little room for error, and keeping this in mind i figure it must be somewhere between 460 and 480. Is this possible? I use (brace yourself) tap water. Yep. I used to use RO water but when we downsized with our move a year ago we never hooked up the RO system, so it's just declorinated tap water, and i have not lost any fish, had any algae blooms, etc. I do use instant ocean for my water changes and this is a quality salt mix so could my high calcium levels be because of this? With calcium levels this high I should not need to add anything in the line of kalk to my top off water, should I? I'd also like to improve my corraline algae (I have some, but i'd like more) is there anything i need to be adding to help this along? it is a biocube and around 20g.

Thanks a bunch

Could your calcium be 460?...yes. Is it?...probably not. It is true that calcium levels in your tank can be replenished through water changes alone (especially on small systems with little or no stoney corals).
My experience with Instant Ocean salt is that it is slightly defficient in calcium (370-400 ppm range) and magnesium as well.

What test kit are you using to measure calcium levels? Some of the test kits available are not the most accurate! In my opinion 2 of the best test kit manufacturers are Salifert and ELOS. Kits are accurate and easy to use (but more costly than API, Seachem, etc).

Also keep in mind that maintaining proper calcium levels is also dependant on maintaining proper alkalinity and magnesium levels. These three things are tied tightly together. Keep all 3 within an acceptable range (calcium 400-450, alkalinity 9-11 dKh, magnesium 1250-1400 ppm) and maintain a decent photo period of 8 to 12 hours and before long you'll have a tank full of purple rock and scraping coraline off the glass twice a week.