"Ray Martini" wrote in message
...
That is totally normal for RO water. It has no buffering capacity at all and
is very acidic. When you mix in your salt mix, this will usually buffer up the
water and raise the pH.

I understand that. That's not the problem. The problem is using it unbuffered
as make up water to the tank due to evaporation. I now understand that it has
to be buffered first. In addition, I am looking at adding calcium chloride to
it to increase the calcium concentration in the tank water (or at least
preventing it from being diluted when I add the make up water).

If you're using the RO water for a freshwater tank (which I do also), then you
have to buff it yourself with Arm and Hammer baking soda. A&H is much cheaper
than the pH up stuff and it does the same job. I'll sometimes use the baking
soda to buff up the water to the same pH as my tank when doing freshwater top
offs.


"George" wrote in message
news:2uo8e.21314$xL4.5103@attbi_s72...
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?