i sue API liquid tests for the basics: ph, ammonia, nitrite and nitrate. When i wan a more accurate test iuse salifert for : calcium, magnesium, alkalinity, phosphates and nitrates. Salifert doesnt sell a complete kit so its single kits only. Elos does also make make some very reputable kits as well. Again, a little more pricey than API but worht it when you need an "exact" reading
I also recently invested in an electronic phosphate meter, which I love to death,. I would like to switch over to their whole line eventually. here is the link for it. Only thing is you still have to buy reagents for them so it comes out to the same price pretty much as a salifert or elos test kit.
Hanna Instruments Checker Colorimeter
If you drop down the menu you can see the other tests that come just like it. My next one will be alkalinity or calcium I havent decided yet.