If you have harder water with a TDS over 100 or plan to use your system frequently (or even daily) a multi-stage system is the idealoption for you! Not sure what your TDS is? Check your TDS here. Customers who have a budget on the forefront of their minds will ask, "Wait, won't a simple DI based system work in a hard water area?" It will BUT you will burn through your resin very quickly which does not make this a good solution for the long run. In a multi-stage system, the RO Membrane serves one purpose: to lower your TDS to a level that will extend the life of your DI Resin. Multi-stage systems are best for areas with high TDS because each filter specifically purifies certain minerals out of the water, they work together to get you that "pure water" which is water that has been stripped of its naturally occurring minerals. If your TDS reading starts to creep above 0 be sure to change your resin to avoid spotty windows or streaks. On the other hand, if your TDS is below 100 or you are just starting out or planning to use your system inrequently, a single stage system may be suitable. Browse our selection of Singe Stage Systems today or give us a call at 862-266-0677 and we'd be happy to answer any questions! If you are simply looking to behin researching or understanding water purification systems, we reccommend checking out our free Understanding Purification guide which will cover choosing a system, your TDS, and much more!