"George" wrote in message news:2599e.1229$r53.744@attbi_s21...
And this is another strange thing in your description.
DI water SHOULD have the same issues as RO or distilled water.
Real DI water should be even cleaner than RO water, so it should
be even more easy to change its pH with CO2 introduction...
If you never noticed low pH of your DI water I would bet it was not very pure
water...

DI resins remove dissolved gases like CO2, so you shouldn't see
the same level of pH drop you see with RO. That was my point.

After couple of hours of aeration the concentration of CO2 in the
water will be in equilibrium, regardless of the water origins...
DI or RO water should have the same pH at this point, if equally pure.
DI water tends to be cleaner than an average RO water, so the pH
of DI water would be lower than pH of RO water, but - again - it does
not matter at all.

Yes. But if you have a lot of CO2 in your indoor air (humans, pets, etc)
the equilibrium after aerating will be on the higher levels of CO2 in the
water, which translates to lower pH levels of the pure water.

Not much I could do about that, since my tank is in the basement.

And this could be the most important reason for your low pH 7.5.
High concentration of CO2 in the ambient air will cause high
concentration of CO2 in the tank water. To increase ventilation
of the fish room is the only way to cope with this issue...
The same high concentration of CO2 causes problems in my tank.

Sigh. Because I didn't realize that RO membranes allowed CO2 to pass through.
CO2 not only lowers pH, but promotes algae growth. When I saw the low pH levels
in my tank (which, in 14 years of running, I've seen it maybe twice), I decided
to test the make up water.

I do not care how much CO2 does RO stop/pass - since I am not using
RO water right from the filter but aerate it in an open bucket...

After couple of hours of aerating, the level of CO2 in solution will reach
equilibrium with the ambient CO2 levels.

With aeration, yes.

Without aeration the same. But it just take a little longer.

Well, after four days with no aeration, the pH was 5.0. That bothers me, if for
no other reason than it makes me wonder what the CO2 levels are in my basement.

You can simply find out using some CO2 meter - like this one:
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=4374263171
You could even think of running automated ventilation system
based on your CO2 level in the basement air :-)
Of course it would influence your whole house ventilation as well.

Yeah, I'll do that as soon as I pay off my car! I used to have access to a
really good probe when I worked at an Environmental Engineering firm here in
town. But since I've been working at another firm out of town, I don't have
that access (this firm doesn't have as much equiptment as the other one did).

You do not need state of the art meter... $50 one would be fine.
I got some used one from eBay made by Thermo Orion about 10 years ago.
Excellent condition, fully automatic, two channels (if you like to
monitor ORP levels as well) fluorescent blue display nicely visible
at night, AC powered - highly recommend: Thermo Orion 525A.

Good luck with your tank. Keep us posted how is it going with pH...