(even though the
heat probably helps dissolve the CO2 better).

Seems right at first:
But no, the colder the water, the more gas it is able to hold in
solution at equilibrium.
Colder water has more O and CO2 or any atomspheric gas everything else
being equal.

If you plan on using a
PH controller then it's a must, otherwise you can always put your
reactor on a timer and save some money.
Hope that helps
Giancarlo Podio

You can use a powerhead to run the system and use a controller with
that instead of a solenoid. The gas will burp out and be wasted is
all.

Solenoid's main usage for practical matters in planted tanks seems
best suited for larger tanks/multiple tank set ups to save the CO2 gas
in the bottle.

Still, planted tanks use CO2 to supply the plants with ample CO2, pH
controllers do not maintain any better control in every planted tank I
have seen or had vs a needle valve and no control(24-7) and what
happens at night doesn't matter much as far as the plants are
concerned. They just want CO2 when the lights are on.

Hydroponic places sell them and ebay etc. 40$ or less is a decent
price for a nice new one.
I'm thinking about one for my multiple tank set up but it'll take
about 6 years to make my $ back in gas savings provided the solenoid
last that long.

Regards,
Tom Barr