I too would lean towards the "natural" sources and then verify through testing that it is ok. That being said one can get a general idea on the health of a body of water by observing what sort of creatures live in it. For example, if you see lots of aquatic insects as well as crayfish then that is a very healthy system. I water out of a 100 gal aquarium and this is a fantastic way of ensuring conditioned water as well as no doubt a healthy dose of trace elements & some fish emulsion thrown in too boot.