Do those leaf tips get wet when you water? If they are getting wet a lot, it could just be from that (If the leaf is touching the soil, or pot, snip the tip of the leaf back so it is not touching. the rest of the leaf will be fine and you won't risk disease from being too moist. Fertilizing with anything other then what is in FFOF soil? Plants look good. I bet that soil is a little nutrient rich for a younger plant but i wouldn't worry too much.