I have a 35 gpd Ro unit (with carbon and sediment filters, no DI filter). I've
been reading up on RO this evening and found several references that say that RO
water can be more acidic than tap water (lower pH) because RO membranes do not
reject CO2. This concerned me since I've been having water quality problems for
some time and thought that it might be a problem with my sea water not having
enough calcium. I bought a Nutrafin pH test kit and a Nutrafin Calcium test
kit. I got some interesting results. For the tap water, I got a pH of 7.5 with
60 mg/L of calcium. At present, the water in my marine tank has a pH of 7.5
(yes, I know it is too low) with calcium at 280 mg/L. The RO water has a pH of
5.0! Calcium was essentially 0. I don't know if it is normal for the pH to be
so low in RO water, but I suspect that adding this water unbuffered to my
aquarium is the root cause of my problems (thankfully, I have quite a lot of sea
sand in the tank, so it does prevent a pH crash below 7.5.). I understand that
RO units do not always result in a substantial drop in pH since it depends on
the amount of dissolved CO2 gas that is present in the feed water. Apparently,
my tap water has a high CO2 content.

This is a good argument for strict testing of make up water, especially since,
in my case, I never tested the pH of the RO water, assuming that it was neutral
(which it is obviously not). I plan to start adding sodium bicarbonate and
calcium chloride to the RO make up water in the future, and aerating the water
for 24 hours prior to adding it to the tank. My question is does anyone know of
a computer program that I can use to make calculations of how much of each to
add? I can do it manually, adding a little at a time, but it would be nice to
be able to calculate it in advance in order to approximate what is needed so
that it doesn't take so long to do or use up the expensive test kits. Another
question. Since I am going to add calcium chloride (turbo-calcium) and sodium
bicarbonate, should I also be worried about adding magnesium to the make up
water?