You can get water softening pillows that you put in the tank that will
reduce the Gh.

I'd heard those contained phosphates, though? Wouldn't that screw
everything up?

Not that I know of.....check with the manufacturers if you are worried.

If it's not likely to be part of the problem, what else would be most
likely
to be causing the problem? And should I somehow try and reduce the
amount
of CO2 my tank's getting? It's a yeast reactor...

Surface movement or an airstone....

So it is too high, even if the fish aren't gasping?

No but CO2 levels & O2 levels are not really linked. If the CO2 gets too
high the fish just get kinda poisoned despite the fact that there may be
high levels of O2 in the tank. You are at the upper end of where it wants to
be. Anything above 15ppm is fine. In fact ANY increase in CO2 makes a
dramatic difference. I've got a full blown Ph controlled Gas bottle system
on one tank & that runs at about 20ppm. On my other tank (which is about 45
USG) I've got one of those Hagen Nutrafin things going. Its way too small to
raise the CO2 levels significantly - I'd be surprised if they were up past
5-7ppm but it has made a huge difference to the plants in there.

rgds

I.

I.