I have 5 koi all the ages of 8yrs. 4 of the koi are fine and eating well. 1 of my koi starting hangin out at the bottom of the pond with his mouth open all the time. It won't eat. I just put in a new pond lily that came planted in little pebbles. My question is can these pebble get caught in their mouth or could it be stress, pre-spawning behavior?