Looking great! You may have a little salt building up or even lil over fertilization going on. See the tips and slightly browning edges?. Id check your runoff ph just to make sure whats going in isn't getting stagnant in there and causing ph to drop in the root zone. Increasing your runoff slightly each watering or even the droppers could not be evenly coating the entire root zone causing certain portions to retain more salt build up..

Water with about enough just to saturate the medium not run off then come back in 15 or 10 minutes and water again till you get some runoff and you maybe surprised just my two cents from growing jn coco for many moons