If they are fed well water, microbes will build up and clog the membrane. TFC membranes are used with well water as they resist the clogging. But they cost more to use. Small systems produce about 10 gallons of water a day. Larger systems can produce fifty or even 100 gallons per day, at about 2-3 times the cost of a simple system.
RO water has a lot less calcium and magnesium in it than the input water.
This makes it much better suited for applications requiring soft water. For the rest of this FAQ I will focus on aquarium applications. In this case, since the RO water is being mixed into the remaining aquarium water that is NOT being changed out, it is not necessary to add any chemicals to the RO water. If you need to use straight RO water in some application, it may be desirable to mix in a little unprocessed tap water. If the pH of your RO water is too high, sphagnum peat can lower it.