i think its ok, i fertilize since they are young like yours, and mine grow like magic beans! soon in a trip ill be able to visit a gigant in the sky! lol!

Its ok, maybe the light is too close to the plants and its buring them, if the light gets too hot and its too close, you are just burining the tips... + give them some dark, thats better for them.
Oh and something else, when you put the fertilizer in, dont spray the plant, and watch the fetilizer that dosnt touches the plant, only the soil.

Keep the growing man!