Ok morts are dead fish ! Foryears i have had 160+ gold fish and 6 koi . I live in the Pacific Northwest and in the winter I get probably a full change of water every week due to rain in the summer it is very hot and windy here my water drops half an inch a day I don’t have a leak and with such heat I was adding water this summer because it was evaporating I’ve never had any amount of Dead like this and they continue to die even in the winter.. I also have a much larger pond that is 30‘ x 40‘ and 5 feet deep that are used to keep water for agricultural use I have 2000+ goldfish in there . And i havent had any morts in there ?
the larger the pond, the more latitude you have with water parameter and fish load issues. The smaller pond is also shallower. Did you do a check to see how much decaying debris/mulm is on the bottom? That's probably the source of any ammonia buildup. You should test the water as noted above; use the liquid (API) test kits as the strips are not very accurate. I'd say you might not have enough filtration or it isn't working well enough to keep up with your fish load. Hot weather exacerbates such problems, including the fact warm water holds less O2 than colder water. What type of water movement do you have? You noted aeration; what kind and how much? Aeration happens at the SURFACE, not with all the bubbles on their way up. If you have no floating plants and/or marginals, this lack of plants will also contribute to water parameter issues. I'd say by the pics, you could use more plants. If you want a 'better' filtration system, search here for 'bog filter' and see if it interests you; most of us that have them don't report such morts, and I can vouch for being 'overstocked', having more/larger fish than you, smaller pond (14x18'x~5' deep) and none of your issues. But I have bog with lots of plants, lots of floating plants, 3+ waterfalls, one aerator, and a partridge in a pear tree. No, wait, scratch that last--the season just kidnapped me for a moment!
Get some tests, think about all the advice you've got so far and compare to your methodology. Another quick thought; any way for pesticides/fertilizers to wash into your pond?