Sounds like there is a real possibility of a farmer spraying week killer on nearby farm fields.
Well there are no farms near me, there is a golf course below me. However about three ravines do bring runoff into the pond...and it had rained a day or two before. I guess it's possible someone did something to their lawn before it rained...I believe he had .3 of inch.
What ever happened the frogs are unaffected, and there are still a ton of small fish looking healthy...so I really can't get a handle on this. Did not see any bullheads dead unless they don't float when they die....pretty sure we still have them in the pond. Hopefully many or most of the BG and LMB survived but I don't really know yet.
To me, a fish kill that kills the largest fish, then the mote fragile fish (LMB/BG) and not bullheads or frogs (which are like a canary in a cave) means depleted O2 levels or Hydrogen Sulfide to me. Frogs and tadpoles are sensitive to chemicals, and can breathe air, so lower O2 levels won't affect them. Bullheads are very hardy, and will survive low O2 levels too.
I would rule out chemicals if the frogs and tadpoles are fine.
There is aerating a pond, and correctly aerating a pond. I say it that way because unless the system is designed and calculations are done by someone that knows aeration, even though there are diffusers in a pond, and air going into it, O2 levels still could crash and fish can die.
Case in point. Client has a large pond, some would consider it a lake. Shallow, mucky and weedy. Their budget would only allow 1/2 of the pond to be aerated year one, the plan was to add the 2nd aeration system year two. Things happened and 2nd system wasn't added. They had a fish kill in the part of the pond that didn't have the aeration system in it....
