ah wrote:

I have a 75 gallon tank with about fifteen rainbow fish. I've ordered
CO2 equipment which should arrive this week.

What's the best way to start this up? Do I just run it for brief
periods of time at first, like an hour a day, and gradually build it
up? I don't want to make any massive changes in the tank chemistry at
one fell swoop.

The best way to build up is to start with a low constant bubble rate.
Start with one bubble per second. Wait a couple of days, then check the
pH to see how much CO2 is being dissolved. Gradually increase the bubble
rate over the course of a week until your preferred pH has been
achieved. Check that once you've set a bubble rate it stays put and
doesn't wander. I have a cheap regulator that changes CO2 flow depending
on ambient temperature (not good). I use a pH controller to overcome that.

The pressure gauge I got has a solenoid, so I was planning to run on a
timer with the light hood, so the CO2 unit will ultimately be on for
about 9 - 10 hours per day.

I use a solenoid linked into the lighting timer on my 16 gallon tank. I
have it on 12 hours per day. When the lights go off, the CO2 flow is
stopped. I don't find that the pH changes much over the course of the
night as the plants start expiring CO2 anyway. I have the bubble rate on
this tank set to 1 every 3 seconds. It's only 16 gallons so it doesn't
require much CO2. My 75 gallon requires at least 2 bubbles per second.

Mostly, I just don't want to come down
after the first day to discover 15 dead rainbow fish.

Start with a constant, low bubble rate and don't let anything or anyone
modify the setting accidentally.

Currently, the tank contains several java ferns, two sword plants, a
micro sword, and an anubias.

My java ferns are always covered in small bubbles of oxygen. How many
watts of lighting do you have there?

Nikki