I take it just the lower leaves are yellowing? Needs a shot of nitrogen. What is the NPK ratio of the fert you're using? You want to use a fert designed for the veg cycle before you switch to 12/12 lighting. It could be that you just aren't ferting quite enough. If your fert brand is the same I just looked up on the web, they have a ton of choices; too many really. This ain't that hard. A grower doesn't need to get fancy to make this work. I've recently heard that Miracle Grow is going to go after the marijuana market. You can get that soil anywhere, but I think what they're trying to say is now they're going to sell their soil in grow shops.
If you're not using a fert that's intended for the veg cycle, you might consider switching to one that is, or buy a fert that is just nitrogen only to use as a quick boost to stop this problem. Those are out there, you just gotta find them. Also, any fert you use you need to make sure that is has as much fert for the money as you can get. So if you you're only seeing, say 5 or 6 ferts as ingredients on the label, it might be smart to try a different one altogether as there's about 16 different nutes plants need to grow propertly. Many potting soils have charges of ferts in them already but that probably won't last long, unless the package specifically says it should be good for 3 months like Miracle Grow. If a grower uses potting soil that should be good for a long time then don't add any more fert unless the plant begins to show a deficiency of some type.