Johnkr, yes the same thing could happen in plant bogs and plant pots....which is why most very serious hobbiests do not use bog filters and do not have plants in their ponds. the organic material in those plant containers also harbor parasites (which cannot be reached using parasite treatments) and anoxic bacteria. And you are also correct in that stocking levels play a huge key in the health of a pond. I have also seen very small ponds with huge koi in them. Honestly, it breaks my heart. Koi can be such wonderful pets and it's just not right to force them to live in sub-standard conditions, just because they are fish. Rock bottom ponds can be kept sucessfully, but the stocking levels must be kept low, and they must be kept clean. To do it correctly, a yearly cleanout is necessary. Totally removing the fish and water and pressure washing all the debris from the rocks is a must. Periodic vacumming in between clean-outs also helps tremendously. And keeping the rock layer very thin so that they can be cleaned is important. If you have 8" of rock, no way can that be vacuumed clean. Personally, I don't want my pond to be that much work. I have found that if pond maintenance is too hard, many people don't bother with it. I am not getting any younger, and the though of yearly cleaning makes me cringe. So I have bottom drains that move all the debris to the filters for easy removal. The bottom of my pond is spotless at all times, and the filters are easily cleaned once or twice a week, a 5 minute job. lefd05, thanks for the kind words and support.

Since you have the pump turned down you could just 'y' off and have part of the water go to a pot filter as described below. For a quick fix filter I have an extra pump and I wrap quilt batting around the end of the hose from the pump, put it in a flower pot that has no holes in the bottom, put some rocks on top of the batting/hose to hold everything in place and set the flower pot elevated in the pond so that the water that overflows out of the pot has been filtered. This has worked better than using the intake of the pump because it doesn't slow the pump. I also have a filter made from a big plastic? pot that has a hole cut in the bottom/side where the hose from the pump attaches. The pot has batting in it and big rocks on the batting. I have the pot tipped so water pours out and this filters the pond. The batting soon becomes a brown/green color and blends in with the rocks. I have also planted plants in the pot (they get big and flower a lot) but it is more work to change out the batting.

Impossible to say other than the obvious. Temp may have played a role but likely a combination of things. Would need to know the temp of the pond and the temp of the source water to even make a guess. My gut is temp wasn't a big factor unless the source water is really something like 40F which is wow cold. Well water can have all kinds of stuff harmful to fish. For example high CO2 which can crash the pH. Different metals, hydrogen sulfide, all kinds of things. I'd be interest to know when the posted test results were taken. Before or after the water change and if after, how long? They don't really make sense. I assume 240 Total Alkalinity means 240 ppm? But pH is only 7? That doesn't fit. For that alkalinity the pH has to be higher, like at least 8.5. I don't generally trust pH tests posted in forums because it's a difficult test to get right. If these test numbers are after the water change it means the well has good KH & GH. If that's true I'd assume the pond had good KH & GH before the change and that rules out pH crash. You could have the well tested for metals, hydrogen sulfide, etc. I wouldn't bother. I assume you've been using this water for the pond before without a problem so even if the water isn't great it's good enough when done slowly. For the record the total number of times I've remembered to turn off a hose in a pond is zero. After maybe a dozen times of my wife saying "do you mean to be overflowing the pond" I finally figured something out...I'll never remember. So I stopped using the hose in this way. Consider using a drip emitter instead sized to whatever pond size you have so you can't add more than 10% of the volume per day. Or turn down the hose to a drip. Let the water change take a week...what's the hurry? 24/7 drip water change is a really old concept in pond keeping that's recently starting to be reconsidered by a few people. It looks very promising and all reports I've read have be very good. Plus we always knew is was good from fish farms that use a flow thru system. So drip is good even if run 24/7. It's way too late now but... moving fish to warm water from cold is OK. From warm to cold isn't good. Just like with people.

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