As fish grow their oxygen needs become higher. In small ponds it is very common for the biggest/oldest fish to die in summer from lack of oxygen. Go outside at dawn and see if they are gasping frantically at the surface.

My condolences on the loss of your koi. I have no helpful suggestions what could be wrong with your surviving fish. Maybe another more experienced koi keeper on the forum has some ideas.

I just wanted to add that I did lose a 14" koi this summer because of the heat and inadequate aeration. Like you, my ponds ammonia, pH, etc all tested out just fine. but we had a snap of weather where temps hovered in the upper 90's to 100 degrees for a week. Found a fish gasping for air and having trouble swimming. Rushed to add additional aeration but it was too late for her. I added extra aeration and the rest of the koi did fine. But, if a fish can die in Northern Wisconsin from high temps and elevated water temperatures causing lower dissolved oxygen in the water after only a week, then I can only imagine it is a real struggle to get your koi through a southern TX summer!

I hope the cooler weather that you are supposedly experiencing (Al Roker said "only" 90's and 100's today for the southern US) will help your fish heal.

Jenny

I'm not a hugely experienced ponder like others are here but I did a lot of research recently when I experienced a total fish loss.
I have a couple of thoughts to share with you-

Check your pH in middle of the day, a few hours after sunset and early morning to see if you may be experiencing a pH crash secondary to the heat. It can affect the fish's resistance to infection (parasitic, bacterial or viral), introduced by goodness-knows-what.

The reddened gills may be a sign of ammonia burn, which seems unlikely as you say the water parameters are good. If there is redness elsewhere on the body as well, look up hemorrhagic septicemia on google to see if that may be a possibility.

Good luck.

You can do a couple of things to add oxygen to the water. First add air bubblers to the pond. The finer the bubbles the better. Agitation is the key. Your fish will likely love to swim through the bubbles.

A quick fix is the addition of a pint of Hydrogen Peroxide to 1000 gallons of water. Since H2O is harmless to the fish it can be added more than once a day. It can also reduce the levels of anaerobic bacteria.

If it doesn't raise the temperature of the water, a fountain with a fine spray can raise oxygen levels.

If none of this helps, you may have a parasite or disease problem. Make sure the fish food has not been contaminated or gotten moldy. Birds and other critters can introduce parasites and other problems.

I'm in Texas too. Please don't blame yourself. This heat caused a whole lot of folks to lose animals, plants etc.... I'm from the Mojave desert. 110 isn't all that uncommon and drought is the definition of the desert. Heat saps 02 from the water. If you live in the desert you're prepared for that. You're not being prepared for that much heat that long HERE isn't your fault. It's a totally different set-up for prolonged extreme, dry heat for many things including gardening, ponding and animal husbandry. I'm really sorry the weather extremes killed your fish.

Thanks for the suggestions & condolences. I finally located someone from a pond nursery several hours away (nothing local) who also suggested a possible bacterial infection. Even though I hadn't introduced anything new in years, he felt that the stress from the heat had weakened them & allowed something dormant to hit. So hopefully the medication - along with a drop in temperature - will keep me from losing any more. I find myself hovering over them watching for signs of anything wrong. The nurseryman said it was just a terrible summer everywhere. So thanks again to everyone from my little corner of the ponding world.

I find a fountain to be very useful in the summer. It adds oxygen, but also increases evaporation and thus cools the water. I had two stock tanks, both with a water fall from the filter. One of them also had a small fountain. The water in that one was about 5 degrees cooler than the one without the fountain.