Do you know what is in your tap water? what is the EC of it? That can be pretty important for more than the Cal Mag reason. Firstly, I'm going to assume you let the tap water stand at least over night ? get the chlorine out? Did you know that some municipalities use Chloramine? won't eveap off like cholrine... you need to know if it's used in your water. If you don't filter it what is the total dissolved solids? It will make a difference in the total amount of fert you can add. If the EC is high in regular tap water what is it high in? could be anything right? Calcium? Silver? etc. Do you have an EC pen? Chlorine is not a metallic element and as such an EC pen will not detect it.