It would be good to know more about your pond etc. Where are you (garden zone, for weather). How long and wide is this pond. ~8 ft high water fall; wow, that's tall! Is there some filtering system? How did you manage to keep your fish alive while you were draining/cleaning? Where did your water refill come from? A pond picture would help.

Depending on the dimensions (of pond And koi), 35 koi sounds like way too many. You forgot to actually say what pH values you found (and how and time of day you measured). I would bet that they are actually OK. AND, while too late now, you probably did more harm than good with the total draining and plant disposal. Again, depending on pond size, three plants sounds like way too few.

I'm in Pittsburgh Pennsylvania The pond is about 10x10 and its 6ft deep. I have a stand alone canister filter with a uv light. I also have a 5500 pump connected too it. I had an extra pump that's smaller so I just leave it running at the bottom of the pond to also help circulate water. I put all the fish in a 250 gallon commercial container and I threw an extra fish tank pump in there from my indoor tank.

The new water came from my hose. I used stress coat for the fish. The fish that are here have been here since day one. I added two of my own last year but that's it. The fish I put in were 9" plecos.I measured at 9am and the ph levels were 8.5 when I measured at 9pm the levels were 9.0. The total water hardness is testing high too.

I don't have a current picture of my pond because I'm at work but this picture was from two weeks ago. I will get one tonight! Thank you so much for the help!

I hope someone else has some info on 'stress coat'. I have a well and don't have to worry about water chlorine-chloramine. A pH of 8.5 and even 9 is OK although you are getting on the high side. Any issues are most likely elsewhere. I have no experience with red algae. Are we talking about stuff that would begin to make you think of beet rather than pea soup or is it only stuck to solid surfaces. If it is suspended in the water, then your UV light should kill it. Again, no personal experience, but I hear that the UV lamps only work for a year or so. Perhaps you need a replacement lamp.

That is still a lot of fish, but I suspect that for the time being (before they grow much bigger), that they should be OK. The major thing that jumps out at me is the lack of any plants IN the pond. The general rule is that around 2/3 of the pond should be covered with plants although that may not be necessary with a deep pond like yours. Obviously, water lily comes to mind.

It is very nice to have such a large substitute container. My uneducated guess is that your pond will probably be or already now is fine. My guess is that the fish that died came from either fish transfer, or stress in pockets of chlorine loaded water that didn't get treated as much as the average water treatment.