In hydro a ppm pen is very useful along with a ph pen or litmus paper. RO filter is good depending on your area. My water sucks so no hydro for me, after 1 week I get this bacterial slime buildup in water, even in the plain water I keep bubbled. I can feel it on the sides (it's slick) and on the air lines so I just can't keep water out for very long.

www.waterfiltersonline.com I think has a ppm pen for like 35-40 bucks. or check the good ole ebay.

peace