I have been lurking on pond boss for about 2 years. I have read probably 90% of every post on here.
My family has a farm pond that is about ¾ acre in size max depth of 20ft. It’s 35 years old fed by water runoff and a field tile. It has CC, BG, Bass GC FHM Most of the bass are small and under sized. We also have a koender’s windmill with only one stone diffuser it was added after a winter fish kill about 10 years ago. It has a lot of FA and a ton of Chara from 6 foot of water up to the banks. The pond has about 1 foot of muck on the bottom witch is mostly made up of un decayed matter. Mostly leaves and dead Chara. The water is very clear you can see down about 3 feet. Vary small plankton bloom if any bloom at all.
I have been trying to talk my family into what they should do to get the pond back on track. Last August someone decided to treat the whole pond in a months’ time with cutrine plus to knock back the chara and FA. Last October we had a fish kill not sure how bad of a fish kill it was. The only fish kill that we have ever had was a winter fish kill witch is when we put in the windmill 10 years ago. I’m wondering what would have caused this fish kill in the fall.

The curtine plus could have done it? I am no expert but if you treated the entire pond and there was that much chara in there and you killed it all in one swoop. You had a D.O. issue I bet. Even with your windmill air. That's a lot of chara killed. Plus they say the weed uses D.O. while it's dieing if I remember right? Lets see what someone else has to say that has a little more insight but I bet you had a mini D.O. crash!

How soon after the chara treatment did the fish kill happen?

Hey Esshup he says above that someone treated the whole pond over the month of august and then in October he had the fish kill. So about a month went by?? Maybe not a D.O. crash??? If his water was fine? It sure sounds like a D.O. issue. No telling for sure!

Apparently you had a fall turnover in October (windmills aren't the most dependable for mixing the water column as they only work part of the time, and the windmill and adjoining diffuser may have been undersized for a 3/4 acre pond).

Since you didn't have a fall turnover kill previously, I'm sure it's no coincidence that too rapid of a weed kill, and subsequent decomposition, exacerbated the anoxic layer that came up during the turnover. Chara is very dense and produces alot of decomposed matter when it dies -- especially the amount you described.

Occam's razor: "It is a principle urging one to select among competing hypotheses that which makes the fewest assumptions and thereby offers the simplest explanation of the effect."

http://en.wikipedia.org/wiki/Occam%27s_razor

Could be a month, could be almost 3 months......

With windmills, the variables are tremendous. Plus with an air stone, when was it cleaned last? As time goes on, the bubbles get larger and fewer..... IIRC, I think it's recommended that you clean even the membrane type diffusers yearly. Those are the easiest. Pull 'em up, scrub them off, drop them back down. IIRC I think the air stones need an acid bath to clean them up. (Muratic Acid)

Thanks for all the comments thus far. The windmill was rebuilt this last spring in may with new airstones and gaskets in the pump.

I use muriatic acid on my membrane diffusers too.