If the calcium concentration is indeed high, 500+ ppm, then the alkalinity will be low. So the first thing to do is to measure the alkalinity. If that is low, then the calcium level may indeed be low, but if it isn't, then that indicates that the test kit reading is incorrect.
The second thing to do when some parameter is outside the applicable range, is validate the test kit that is being used. This involves using it to test a water sample of known concentration / amount to see if the test kit matches, or getting a second (and even a third wouldn't hurt) test kit from another source to determine the value. It is very likely with a high calcium reading and suitable alkalinity that the calcium test kit is faulty.
Third, if that is the true reading and alkalinity isn't low, then check the specific gravity. An above average value (1.024-1.026) will allow a higher concentration in the water. Therefore, if it is too high, around 1.030, then that may allow such an elevated level. In this case, slowly bring the specific gravity down either by water changes or removing tank water and replacing with pure water.
And finally, if you reach this point then simply don't worry about it. A high calcium level is not a problem as long there is not difficulty in maintaining alkalinity. May be back off a little with calcium supplementation and allow it to drift down naturally, or perform a water change or t[w]o. But don't be too concerned about it.
