PJ, you got to get one of these...this is an in-expensive one...
http://cgi.ebay.com/NEW-Accurate-DIG...QQcmdZViewItem

as Z stated above, you have to have good ph...you can watch your ppm's by not over doing mixes, but ph can be tricky...shop around, but you won't find a better deal than the one above to get you started...If you miss this one, check sellers other items and he might have a full stock of them. hope this helps. lw