I like the Red Sea test kits personally, but I also use the API ones as well. I find both to yield accurate results as long as you follow the instructions, but the Red Sea tests give results on much smaller scales so you can get a more pinpoint reading. Normally I do my regular testing with the API kits because it is faster and they are cheaper, and I use the Red Sea kit to double check any unusual readings. (Specifically the kits I use are Red Sea KH, Ca, Mg, NO3, PO4; API pH, KH, NH3, NO2, NO3, Ca)