Hi Lynn! Here is some good info for you to read: http://ag.arizona.edu/region9wq/pdf/nv_selectRO.pdf and How to Select a Reverse Osmosis Unit for Your Aquarium.

From when immersed in salt water hobby; remember that the capability to back-flush is good because it will allow to clean and extend the membranes life. Also, some units had built in pumps that think would make the system more efficient.

Here are some online sellers I used to buy from and are OK: Dr. F & S & Big Als. Hope this helps and good luck !