Chuck Gadd wrote in message . ..
On Wed, 11 Jun 2003 19:36:07 -0700, "Djay"
wrote:

I recently began using a pressurized CO2 system that keeps my CO2 at 6.8 in
my 75 gal tank. My tap water is around 7.6 Ph and after several days of
aeration it remains at 7.6 Ph.

One day, I can foresee needing to do a large water change... possibly 50%
for whatever reason... I figure my fish would appreciate me avoiding
dangerously large Ph swings that would happen if I just dumped 35 gal of Ph
7.6 into my 6.8 tank.... I have a couple ideas:

I've done large water changes without any problems. The pH shift
doesn't cause any problems. And I think the pH shift will be MUCH
less than you think.

Consider:
I'm guessing your KH is around 4 degrees.

70 gallons of water at 19ppm CO2 = pH 6.8.

After a water change, you'd have
35g at 19ppm, and 35g at 3ppm which would average out to 11ppm,
resulting in a pH of about 7.05.

So, you've got a pH shift of only 0.205.

No problems!


Chuck Gadd
http://www.csd.net/~cgadd/aqua


I agree with Chuck here and having done monster water changes for many
years, they don't have any effect on the fish, Discus, Cardinals etc.

Your CO2 system will lower the pH down fairly quick, maybe an hour or
two or so.

Regards,
Tom Barr