One our our older koi (she’s about 13) is acting a bit different. She seems lethargic, lips and gills moving constantly. Seems to be hanging out by the skimmer and waterfalls. It seems to me to be oxygen deprivation but I can’t find a reason why. Bought a pond test kit and checked ammonia, nitrite and PH levels. All are good. PH was slightly elevated but not off the charts. Pond is being aerated very good between the aerator and the waterfalls.all plants are spring to life. In fact some are in bloom already. She is the only one that is acting different. No obvious signs of injury, lesions, droopy fins etc…. Noticed all this last week. Not sure what to do. Any suggestions would be greatly appreciated.