2 Digital water purity testers (test TDS) for $10:
(I don't think I'm breaking any rules b/c this isn't plants...but maybe I'm wrong?)
Its a strange website with a strange sense of humor, but they're reputable (I've ordered from them before). If you don't order by midnight though, they won't be selling those testers anymore. They only sell 1 thing per day, and it changes at midnight every day.