Thanks to Kush and Leigh, very much. I drained half the tank (about 25gal)
and slowly added fresh water, not to change the pH any more rapidly than I
had to. The pH is at 7.3-7.4 now.

I received my champagne yeast in the mail. I looked for some recipes on the
ng and web. This is what I ended up doing:

1 1/2 cup sugar in water, not quite hot enough to be painful to the hand.
Not quite a teaspoon of yeast. Gently shaking until all sugar was
dissolved. This is in a 2L soda bottle. I put on a cap tightly and after
about 1.5 hours I could tell there was significant pressure in the bottle,
so it was producing.

I now have the bottle hooked to airline hose (silicone needs to be ordered,
using regular airline hose for now) running through a gang valve with check
valve, and into the intake of my power filter.

This morning the pH is neither down nor up. Still about 7.3. I'm guessing
(or think I've read) that I might need to use some baker's yeast too.

Any suggestions? Oh, I have the bottle sitting on a vent from the light, to
warm it up some. I will be keeping a close eye.

My fish seem to be doing great, and I am very thankful for the help.

bob allred