Not sure what is wrong, only 1 girl having problem.

So I have 2 plants going, same set up, same soil, same watering, same everything. I have not added any nutes yet. Both are still growing at a reasonable speed, neither is stunted. One has lots of yellowing and leaves are getting brittle. Not sure what the problem is after a bit of looking around/research. Could be a few different things. Looking for help narrowing it down a bit please.

somethings eatin it. thats what it is

I havent had this problem indoors
However outdoors I have, so Im going to have to go with stinkmeaner on this one

pests, put a strip of duct tape down sticky side up and if you catch any bugs, you need to use a pesticide

There are gnats that came with the soil, they must be the culprit I guess. I'll get some sticky strips today. Thanks for the input.