Top feeding is when you suck up some nutes from the pail and squirt them around the base of your little plant to feed it until the roots emerge from the net pot.
I think I'm seeing the root of your problem now. No roots! And no food for the sprout. Amazed it's still alive at all.
I have a 60cc syringe with some tubing on the end for sucking up samples for testing or top-feeding new clones. A good old turkey baster works well for that too and is easier to get cheap at most stores that sell kitchen stuff.
Shouldn't spray anything on your plants if you don't have to. It's most likely that's what causes those lower leaves to brown like they did at the ends. All makes sense now.
And add some more balls to block the light better too if you can see into the pail thru those slits in the pot.