Copied and pasted from my answer to your duplicate thread:
That 75 gallon tank was about 20 inches high and 18 inches front to back depth. The light you had gave you medium light intensity, possibly low medium light. As a result you didn't have the light driving the plants to grow so fast that they needed CO2.
If the 125 gallon tank you now have is also 48 inches long, but 24 inches high and 24 inches in front to back depth, the same light fixture probably gives you low light, and less than low light near the front and back glasses. If you added another of those same light fixtures - 4 x 65 watts, with one placed close to the front glass and the other close to the back glass, you should again get medium light, possibly a bit higher than on the 75 gallon tank. So, that should get you about the same conditions you had with the 75 gallon tank.
The watts per gallon isn't a significant number here. What is significant is how many PC bulbs are above the substrate at every point in the tank. The 4 x 65 watt fixture puts two bulbs above the area directly below it. Adding a second fixture lets you provide that same light over a larger area of substrate.
If you replaced the lights with two 2 bulb T5HO fixtures, spaced around 8 - 10 inches apart, you should also get medium light intensity, and pretty uniform over the whole substrate.