Ammonia is the first thing to kill your fish. You are going to have to have that one. If you don't want to spend a fortune, I recommend "Aquarium pharmaceuticals" saltwater master test kit. It does PH
, ammonia, nitrite, and nitrate. These are the four things you absolutely need when starting a tank. It runs about 25 dollars and will do about 75 tests or so per type. You can find it at Petsmart, Petco, or most lfs
that carry saltwater fish. It's user friendly.