To expand with a bit more detail on OG's response: "hardness" or GH is a specific test of divalent cations (positively charged ions), and more specifically of calcium (Ca++) and magnesium (Mg++) ions, the two cations which affect soap bubble production in water. TDS is a measure of all ionic dissolved materials in the water, monovalent (as sodium, Na+), divalent (as mentioned), whatever; it includes both cations and anions (negatively charged ions: chloride, carbonate, nitrate, etc.). Anything ionic dissolved in the water will register on TDS, GH is quite specific and far more limited. But GH is the parameter commonly refered to when discussing source water for fish.
If you can get access to the February issue of TFH (Tropical Fish Hobbyist) magazine, there is an excellent introductory article on "The Ins & Outs of Osmosis" by Mark E. Evans which should be required reading for all hobbyists IMHO.
