Saw this on ebay HM Digital TDS-EZ Meter/Tester, Water/ppm/Purity/Filter - eBay (item 280349439863 end time Apr-18-10 15:58:02 PDT) Can I use that for hydroponics too?
No idea, I use this one: HANNA Primo TDS Meter, PPM Tester, Conductivity - eBay (item 270336297150 end time Apr-22-10 10:26:45 PDT) If you look around you can get the Hanna one for about that price with the 1382 ppm calibration fluid as well. EDIT: Actually that's the one with the free calibration fluid lol.
His feedback on that meter is good. It is a cheap meter, therefore you cannot expect too much from it. I would get some cal fluid and watch it carefully to avoid a catastrophic error. I always buy the best tools that I can afford, and have never been sorry.