Hello there Tropical North,
I use the Saltwater version of this test kit. You cannot use this for testing salt water as the contents of the bottles are slightly different and the ammounts you use are also different for testing saltwater..I know how you feel, i used to use the freshwater kit when before i went across to marine keeping and ended up buying the other version..The only test kit by Aquarium Pharmaceuticals that tests both is the Freshwater/Saltwater Professional Master Test Kit..
The way i looked at it is that when i test my water, i want to be completely sure that i am getting accurate results, and using a test kit that is not designed to be used with your water medium cannot deliver that..
Hope this help..
Niko