Why would you really need high CO2 when you are only using 1.4 w/gal?
Will adding CO2 help?
Sure.

Will it make the system more or less stable?
Most likely more stable, thus............why destabilize it?

That's a very obvious question.

If it's for test purposes, think about the test first.
Think about what your question is.

That's a different question than whether it's more stable without CO2.

Why add so much light in the middle of the day?
4x54w T5's can grow any plants and that will reduce the trimming and CO2 demand and growth rates.

Why use all 6 if that is really your goal?
You can measure the light in aquatic systems easily with a light meter and no, after after 1-2 hours, the light is far more intense than folks are assuming.

800-1200micromols for about 10 hours out of a typical 12 hour cycle.
Far above the levels we have and that aquatic plants demand(which is very low).

You might also try running one back of the T5's for 3 hours each, front to back etc.
That would give you good spread and lower light intensity.

Thus less growth. less trimming and less CO2 demand and a more stable tank.
While also providing with thicker growth and foliage.

Also, how is CO2 for 5 vs 8 hours better/worse?
CO2 is cheap, we add it to increase growth rates namely.
You might save 7$ per 20lb tank based on a 20$ refill cost.
A 20lb tank ought to last 2-4 years at least on a 75 gallon tank, you save: 3.50 to 1.75 per year.

I guess.

Tropica showed that low light tanks gain benefits from adding CO2 as well.
Lower light makes the CO2 demand go way down, so supplying enough CO2 is a lot easier.

If you reduce light down far enough, then you no longer "need" to add CO2 at all, is that somehow of interest to you?

If so, try some non CO2 tanks.
They grow plants and reduce the trimming efforts etc.

Ask yourself why you add CO2 to begin with, most plants do pretty well without adding it, they just grow a lot slower...........



Regards,
Tom Barr