I use API for alk (KH) and Ca testing. You add drops until the colour changes. So that might work for you, if you can detect a colour change.
For phosphates (P04) I use the Hanna digital checker. Very accurate, but kind of expensive.
Don't bother with pH testing. Not needed for salt water.
Don't have a good suggestion for you with nitrates (N03). All the ones I know use a colour comparison. Maybe take to your LFS for testing.
Once you are fully cycled, you won't need to test for Ammonia on a regular basis. Maybe take to an LFS for that if you suspect it.
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