Does seem odd. Any particular time of day or all the time ? Any plants or algae die off recently? Hot, still weather? Assuming you are not overstocked? Any other recent changes or possible stressors such as pollution or run-off of some sort?
Could something like chemicals get blown into the pond .I know weed killers and bug killers can get blown around in the wind .Plants give off oxygen during the day but use lots of it up at night .You say bubbler is that from an aerator and how big are the fish and how warm is the water .If they are gulping for air in the morning then it could be the plants
25 fish in 700 gallons would call for extra aeration. I would turn on your other waterfall. Have you tested your water, is the ammonia high...high ammonia burns gills. Test your PH also, as a high PH makes ammonia more toxic. After you test the water, I'd do a partial water change and add de chlorinator....not a huge water change.....I'd do small water changes more frequently and observe the fish and see if they improve.
Could also be nitrite poisoning. That causes brown blood disease and makes the blood unable to take up oxygen so the fish suffocate no matter how much air you add to the system.
You need to test the water with liquid test kits for ammonia, nitrite, nitrate and KH. That will give you an idea of how safe your water is.
pictures pictures and more pictures you can have twelve bubbles if they are an open ended air tube with large bubbles that would not put enough o2 in the water and if the water fall is a trickle falling a couple inches that two would be lacking . Need a little more info or a pic or two
8 is fine for pH. Don't be concerned about a specific number, just that it is stable. There is no perfect or ideal pH for koi or goldfish. They easily adjust to a wide range of ph levels and anything between 7 and 10 is good.
PH is not that important it's more of a problem if your ph fluctuates quickly. so if you find your ph is high or low raise or lower it slowly. That is the main reason small ponds have more of a struggle temps can change to fast or ph or even ammonia. when you have more water it is hard for anything to fluctuate quickly , thus stressing out the fish less