Calcium levels in marine aquarium seem too high, for no obvious reason!

First up how old is your tank?
For the not so old tanks, your bio life may go thru a time of exploding populations and most of our cycling bacteria will produce its own basic yellowish nitric acid!
It’s needed especially for oxidising by nitrate reducing bacteria.
If your media is mainly calcium based those little guys acid is breaking it down and releasing its make up into your water, the process is so slow you will never see the calcium media reduce in quantity.
Not saying its your issue but have seen this many times.
With one of my early ones of near all coral rubble, the calcium went well above 700! It falters in time, some times in weeks some times in months!
The substrate is normally the issue, till it begins to block with rubbish and the bacteria community reduces.