I'm running a Water General unit. The RD-102. I have it just about 2 years now. It came with everything needed to hook it up under the sink, including the waste drain tap. I didn't hook mine up under the sink, I placed it in the garage. I also got a dual in line TDS meter for it, so I can read the incoming and out going.
They work by forcing the water through a RO membrane. The RO membrane catches all the unwanted stuff. The RO membrane is what makes or breaks a unit IMO. The water that makes it through the RO membrane then goes through the DI resin. The resin then traps the little bit that gets by the RO membrane giving you 99.99% pure water. Lots of goodies and add on pieces to a RO/DI unit. One thing to think about if your interested is a pressure reservoir tank. This will give you the RO water for drinking, cooking, ice maker. Then you will have another output for the RO/DI water for your tank. Booster pumps to increase efficiency of the unit, ect. NONE of which is needed to make good water for your tank. Just options in how deep you want to go with your water unit. HTH.