Jason wrote:

I am going to start injecting CO2 and wanted to make sure that I don't mess
anything up too bad. I plan on doing this with a 1 gallon DIY CO2 reactor.

I have a 50 gal (48x15x16) tank with the following parameters
120w of NO fluorescent
PH 7.5
KH 2
I think this gives me a CO2 level of about 2.5-3 ppm.

According to Chuck's calculator
(http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm), this gives a CO2 level
of about 2ppm, which means you probably measured pH late in the day, after the
plants were sucking CO2 out of the water for a while. If you measure again first
thing in the morning, before photosynthesis starts, you will probably get pH
7.3. This yields a CO2 concentration of about 3ppm, which is the level in an
uninjected tank in equilibrium with the atmosphere.

My question is should I raise the KH before injecting CO2. I plan on raising
the KH with baking soda. I think I understand the relationship between these
properties but would like to make sure. From my understanding if I raise the
KH to 4 it will bring the PH up to around ~8. And then injecting CO2 will
bring it back down.

At KH4 and a CO2 level of 3ppm (before lights on), pH will probably be about
7.6. After the plants start sucking the CO2 out of the water, it will probably
rise up to 7.8.

Now, regarding your CO2 plans: DIY CO2 works great. But to have a significant
impact on a 50 gallon tank will take a lot of CO2: at least two, 2-liter bottles
working in tandem (Start bottle A on Day 0, start bottle B on Day 5, change
bottle A on day 10, change bottle B on day 15, etc.) This works fine, many
people do it. If this sounds like too much work, consider pressurized CO2
instead.