Total Dissolved Solids. It's sort of a conglomeration for dKH, gH, pH, conductivity, alkalinity, and buffering ability resulted by the overall mineral content of the wayer. Basically a count of everything that gives the water it's elemental parameters. Your geode will add to these levels. Depending on the current TDS level of your water, you might or might not be able to measure any difference in your elemental (as apposed to biological, i.e. ammonia, trites and trates) parameters. To simplify, the lower your pH, the more effect the geode will have on your water. I can't tell you exactly what the lowest pH you can have with no measurable change would be without knowing the exact molecular make ups of your geode and your water. As a guess, I'd say you wouldn't notice any change unless your current pH is less than 7.5 - If your pH is down in the 6's, I imagine you will detect parameter changes fairly quick.
If you do have pH <7.0, and really want to use your geode, you can, but you'll need to do water changes a little more frequently to combat it's water hardening effect.
FYI, TDS is a measurement used almost exclusively in the saltwater reef side of the hobby, as it is vital to reduce all the water's components to zero, allowing hobbyists to add carefully measured doses of some of those components back into the water so that they do know its exact molecular make up, and adjust accordingly. Very few freshwater fish require this extreme knowledge of water chemistry... discus are the only ones that come to mind right now. As long as you're keeping normal, common tropicals, the only parts of the TDS levels in your water you need to worry about are pH and hardness.
It *is* helpful to have a brief understanding of what TDS means, obviously, just in case some chemistry nut shows up and mentions it.
That's a cool heirloom to have, btw. Good luck with it.
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