No, there is no real way to check what chemicals are present in your water with just that pen. You would have to send it to a lab to get it analyzed or get some stuff to check it yourself. But as a general "rule of thumb" tap water is ALWAYS high in cal & mag. This is why you don't need to add cal mag to your pure vida nutrients, it is already present (in probably very high levels. tap water is usually very hard water as well) in your tap water. It seems to be causing a build up of those nutrients in the soil, locking them out.

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Keep the frothing pigs at bay.