RO and pH issues.

You don't need to buffer the RO water you add for top-off water. RO water
has basically no buffering ability, so when it's added, it will instantly
take on PH of tank water. It will not bring your PH down. Something else
is bringing PH down in your tank (or simply keeping it low). You might
check your salt mix first and your SG, then check your Alkalinity. Adding
buffer and calcium to your freshwater top-off water is only asking for
trouble.

-Todd
"George" wrote in message
news:lyA8e.22828$Bb3.8461@attbi_s22...

"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?