What Big boy said.
First (ie bottom) large fan leaves on a plant will typically turn yellow and die. That's normal, especially for plants restricted to small containers. Late in flowering, fan leaves will also typically turn yellow and die.
In either case, you want to pluck yellowing leaves off, ASAP, because they just attract insects, provide a place for mold to grow, and (if high on the plant) block light.
As a second issue, I see your leaves are starting to curl up a bit. If your temps are OK, that may be from overwatering. You shouldn't have to water more than once every few days, especially not for small plants in veg. If you've thoroughly flushed the plant with water to normalize your (probably non-existent) pH problem, you may need to add some nutes, because the flushing may have depleted pre-existing nutes. Don't add much.
Lastly, you didn't ask, but your small plant isn't going to be happy for very long in a solo cup. Unless you want it to be stunted and scrawny, I'd suggest moving to a larger pot. Even the cut off bottom of a 2 liter soda bottle will make that thing much happier than a 12oz cup.
