Watt means essentially nothing btw, its more about if your lighting is t5 or t8 or led or what have you, and ultimately and at the core of it all, its about the level of PAR your lights are producing.
Different types of lighting can achieve greater PAR values while using less Watts, making the idea of watts per gallon essentially useless. So, find out what kind of bulb your fixture is, and I'll assume that its either that the lights are next to each other and not one in front of the other right?
I forget offhand how long a 125 is and the exact dimensions of that kind of tank, but the most important thing for you will be to get no more than medium light and have it be evenly distributed across the tank. Any more than medium lighting and you'll be needing to inject co2 into the water, there is no additive that will be worth your time when we are talking a tank this large, as there is nothing that is anywhere as good as straight co2 injection. Liquid carbon additives are okay but they aren't a replacement for the gas in the slightest.
Next, you'll want to be sure you know about how to fertilize and feed your plants nutrients for them to grow. The biggest misconception about planted tanks IMHO is that you just need light to grow them and that's it. The truth of it is that a planted tank is a giant balancing game between available nutrients (marco and micro, macro being nitrogen, potassium, and phosphate, and micro being trace elements like iron and calcium among other things), available carbon, and the amount of light. Putting it simply, if you have an off balance of any one of these three factors you're going to be dealing with issues and not be truly satisfied with the tank.
Read up on it as much as you can, The Planted Tank is a good place to look, as well as Aquarium Plant Central, and I'm sure there are many others that exist that are good to look at. If you want names to think about in the planted aquarium world, Tom Barr is one I recommend looking into, as he basically defined how to properly grow plants in carbon injected tanks with ridiculous amounts of lighting. There are plenty of people out there that can talk the artistic side of planted tanks but I feel few of them have the science behind it down in the same way Barr has it.