What is it? Why does it help?

RO water is Reverse Osmosis water. What is this? It is water that has been forced across a semipermeable membrane, stripping out unwanted solids or liquids. The solids that can stripped are the ones we worry so much about: calcium, magnesium, and carbonate. Ideally, the RO unit also has active carbon attached to it, to neutralize ammonia and chlorine. Once water has gone through a system such as this, it can be essentially ‘pure’ water. I say essentially ‘pure’ because (sometimes) gasses can cross the membrane, leading to contamination. Chloramine can do this, which is why it is critical that the carbon on the unit is active.

Ideally, in a perfect world, water has the ‘perfect’ pH of 7.0. This pH is acquired without buffering. Without buffering, water is also susceptible to any and all chemical traits of a substance that may be dissolved in it – like organic acids or nitrate (nitric acid). Once you put just a little acid in ‘pure water’ – you immediately will get a pH crash. This is why RO water must be reconstituted (adding back minerals, trace elements, and buffers).

RO water often is not 7.0 when pulled from an RO unit, and actually can be quite startlingly acidic. This is because of the ‘tightness’ of the membrane. The electrons can actually be stripped off the water molecule, leading to a net ‘H+’ charge: acidity. Over time, the more the membrane is used, the tendency to run acidic will lessen, and a ‘well used’ membrane will actually have water closer to a 7.0. RO membranes should be replaced once a year, perhaps sooner depending upon water hardness. It is easier on RO membranes if you can have a water softener prior to the RO unit, helping to remove some of the calcium and magnesium.

Because all the minerals, etc., have been stripped, the RO water has no hardness. None. There is no buffering, no calcium, no magnesium, no sodium. This results in 0 (zero) degrees KH, GH, and no TDS (total dissolved solids). You may think this is the perfect medium for fish happiness (pure water!) but in reality, it will make your fish very stressed and unhappy if you choose to put them in this.

In order to use RO water for an aquarium, you need to reconstitute it. You can do this by one of two ways:

1) By adding back in tap water to a specific ratio that you decide
2) By using water constituting products (such as Seachems) or using a mixture of baking soda and epsom salts (I don’t use these, so I can’t give advice. Perhaps someone else, like Dwalin, will chime in).

Adding back tap water…

Adding back tap water is by far the most economical. Remember you will still have to add in a dechlorinator (for chlorine) or a dechlor with de-chloramine (for water with chloramines). I don’t recommend adding back tap water if your tap water has free ammonia or nitrites in it (as mine does)

RO water is a clean slate. If you want to add back a specific amount of KH, all you need to do is figure out the KH of your tap water. You then add it back in a ratio to your RO water. For example:

If your tap water had 200 ppm KH, and you wanted your tank water to have a KH of 60 ppm (which will result in about a pH of 7.0) you would add roughly one third of your tap water back to 2 thirds RO water. This isn’t precise, and it would take a little experimentation, but it can be done easily and cheaply

Normally, GH and KH walk hand in hand. If you adjust back to get at least 3 degrees of hardness for KH (roughly 60 ppm) your GH should be fine as well. However, there are people who have unearthly high KH, with little to no GH. At this point, the tap water alone will not reconstitute the GH, and you will need to do additives.

By using water reconstituting products…

When I first started using water reconstituting products, I first started using Kent. It worked well, but the KH and GH were tied together, and you had to do a lot of premixing as it was a powder, and not a liquid. When you are doing water changes, and relying on this powder to dissolve so your RO water can be correctly buffered… so you know if you buffered it enough…. then test it… then add more…. you get the idea.

Seachem has a liquid alkaline buffer which affects KH only. They also have Equilibrium (a powder) that affects and increases GH only. GH, while important for your tank, does not throw off a fish like KH will, and can be added slowly back to the tank during water changes. The liquid alkaline buffer, however, is nice for two reasons:

1) Each 5 cc will raise 5 gallons of water almost 10 ppm KH exactly.
2) The liquid can be drawn up in a large syringe for accurate measurement.
3) Mixes instantly.

By controlling the exact amount (and I do mean exact) you can control your pH perfectly. Once you find the KH that gives you your ideal pH, it is a simple matter of calculations with this product. It’s a breeze.

The GH supplement (Equilibrium) is much the same way. 1 T per 20 gallons yeilds a 3 degree hardness increase.

You will also need to add trace minerals (for fish) and trace minerals (for plants). It is your call on which products you want to use. There is no mystery to that part (thank goodness).

Epsom salts and baking soda…
I know that this is a very effective, and inexpensive, way to reconstitute RO water. I just have never done it because I fell in love with Seachem’s products, especially the liquid KH. Hopefully Dwalin or Off-Ice (I think Luv or Pass do this, as well) will stop by and describe how to add this back in,
as it is effective and a good method to know.

As far as renting/purchasing/owning RO units, I can only say this: I used to purchase RO water from a LFS for 30 cents (US) per gallon. I would go through 70 gallons a week. After a year of indecision, I finally called Culligan Water Systems here in Iowa. For 20 per month, I got a small RO unit which produces about 15 to 20 gallons per day – if I remember to turn over the hose into new buckets. The rental cost is a stunning 20 per month (again, US) – I easily went through that in one month by purchasing. Culligan does all the maintenance and replaces the membrane for us each year, free of charge. Installation cost 80 dollars, however, the first 3 months of rental were free. The unit is amazingly, amazingly small (I again was shocked) and very convenient to have. I didn’t have to do any more late night runs when I needed to do an emergency water swap out. I have RO whenever I want.