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yea you need a tester, also dont forget to test your water before you test it with the nutrients in it.if your goal is say you want 600 ppm and your water is 250 to start out with then you have to factor that in.
also i don't recommend a cheap one, look and read reviews and see what everyone likes. i use one that i never have to be calibrated, i save on buying the fluid used to do so and it has been very good to me and very accurate.