Mark,
My opinion is biased on saltwater test kits.
I like and use Salifert for the most part.
This site sells them under the dry goods section, at a very good price of $9.99 by the way.
If you've got the bucks - I'd go with alkalinity, calcium and pH.
These 3 measurements are normally best looked at as a group.
Each can and will have an affect on the other two as you make adjustments to the tankwater.
A high alkalinity can be caused by several things, and I'm sure I'll miss a few.
Poor quality dry saltmix.
Using tap water that is very high in excess minerals and carbonates for saltwater mixing.
Topping off with tap water that contains monerals and carbonates.
Allowing the calcium to drop off significantly.
Adding pH buffers or alkalinity building products in excessive amounts.
Not doing water changes with good quality saltwater mixed to a proper salinity, and allowing the tankwater chemistry to become out of balance.
Low fish loads, low organic acid levels and little if any algae growth - combined with overdosing of buffer products.
If the pH does not drop from these things - the alkalinity will not be required as much to "buffer" the tankwaters' pH.
I'll guess that your calcium is low, and you top off with tap.
But I'll ask if you use any buffer products too.