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      For the majority of Reef Keeping Hobbyists, I feel this one piece of equipment is a must have. With the quality of water that we must maintain within our aquariums to keep our inhabitants thriving, it makes no sense to use anything less than the purest freshwater when we mix up a new batch of saltwater. There are very few of us that get the quality of freshwater we need delivered straight from our household taps. In the off chance you may be one of those lucky few, I would suggest you get your water report which is freely available from your water department, you may even find it published online.

Local Drinking Water Reports

Reverse Osmosis = This is a purification method for tap-water. Prefiltered tap water is pushed through a reverse osmosis membrane. Water that makes it through is considered pure, while water that does not, is sent through a special tube and is rendered impure. As it relies on water which is able to pass through the membrane, it also generates a large quantity of "waste" water which cannot be used. This is one of the best, but slowest methods of tap water purification.

What constitutes a Unit? - As you can see in the above photo, an RO/DI unit can consist of several chambers, each of which performs a filtering method. Depending on the models purchased, they can come with any number of "stages", some units even have UV stages for sterilization, but for our purposes, a simple "home use" unit will do just fine. As such, I would buy a unit that has at least the following four stages built into it.

Sediment Filter - Removes the largest particles from the water, which helps to prevent the finer filters next in line from becoming clogged with material.

Carbon Filter - Removes by adsorbtion dissolved organics and chlorines which would damage the reverse osmosis membrane.

Reverse Osmosis Membrane - The actual membrane that allows only certain sized molecules to pass through it. While it does a good job of purifying the water, it does allow compounds / molecules of substances that should be removed, which is where the dionizing filter comes into play.

Dionizing  Cartridge - Contains a resin which adsorbs those elements that were small enough to pass through the RO membrane with the water molecules.


- When purchasing a unit, name brands are not very important. There are only but 2 or 3 manufacturers of the RO membranes, which are then bought by the makers of RO units to put within their units. Brand "X" most likely has the same membrane as Brand "Y", the housing for the cartridges are usually the only difference, which means a 100 dollar unit will most likely perform just as well as a 400 dollar unit.

- Buy a unit that produces no more than 75 gallons a day. The larger units use a membrane that is not as "fine" as the smaller units, thus allowing more pollutants to pass through them. You will get a more pure end product with the smaller units.

- Buy a TDS METER, this is critical in determining when you should replace the cartridges and resins. Do not go by time passed as is usually recommended, Your city's water and pipe conditions may clog up and use up the filters alot faster or slower than another area of the country.

- Use a pressurized storage container for the storage of the water produced. If not, then get a shut off valve to prevent overflow of the produced water. I do not know of anyone who did not have such devices that did not sooner or later end up flooding a room when they forgot the unit was running.

- These units create alot of waste water, please try to find a use for it, such as watering certain plants, doing your laundry, giving the dog a bath or any other use that you can think of that will put that water to use. Keep in mind though, the waste water will be very "hard" water and not suitable for drinking.
This mirror is being hosted with the permissions of the original content creator for preservation and educational purposes.