I think you need a new test kit
* edited *
I was a little hasty there, sorry.
If you are using a salt mix that uses Borate to artificially raise ALKalinity then your ALK
reading is possible. It is not possible to keep Ca
supersaturated with a true Carbonate Hardness of 18 at reef PH levels.
I believe your either have a defective test kit (Ca
) or you are using a salt mix containing Borate and you may never know what your Carbonate levels really are.
If your corals are growing then you obviously have nothing to worry about
Borate is misleading and I do not recommend using it as a buffer. It can supress Carbonate levels and prevent calcification in corals, stunting their growth.