Tds meter

I would go with something from HM Digital. You can get one for under $25 and they have proven to be accurate.

+1 HM digital.

HM TDS Aqua Pro is what I just picked up its ATC and has a digital temp on it also. avoid the TDS EZ its not ATC I made that mistake buying it. Just buy one that is ATC there is a few other good ones HM make. I got mine for like $15.00 shipped on ebay.

HM Digital TDS-3, TDS-4TM or AP-1, all are around $25 most places. Be wary if you find it for much more than that on ebay, there are a lot of imported knock offs that look but do not oerform like the real thing.

Hand-Held TDS Meter | Melev's Reef <----this one.