With out a water analyis show water hae rdness and calcium levels it is merely a gamble without RO. However with a TDS as low as 100 I would not be cocerned with calcium carbonate levels or Excessive maganes, excessive sulfur, excessive copper and lead. In general though an RO filter is not needed with a tap water TDS that low unless you live in a hi ome with copper piping built before the 1980's. If you liver in a home such as that your copper levels will be too j high unlseess your tap water has run a while after have ing set without drawing water for a period over 4 to 6 hours or so. Considering most MJ nutrient manafacturers actually supply to little calcium and no carbonates to their nutrients as mentioned some additional is usually need when using RO water.