i usually top off with plain water and try to change res every 7 days. If i know its gonna go longer than a week i give it a little nute bump in the middle depending on, like massproducer said, the magic ppm's.
i dont know what brand it is , it just says tds tester, microprocesser series...its tan and about 5in long with 2 prongs on the end. it goes fro 0-1990PPM's...thats all i know