Help.
Background first. Pond is in SE Lower Michigan previous owner had the pond dug in a swamp area in my back yard, 300 feet long and 250 feet wide about 15 feet deep, the previous owner ran out of money to keep digging to get to good soil, the bottom and sides are swamp peat? Very black with springs all thru the pond. I have owned the property for 21 years and always had suspended black peat, no problems and kept it stocked with BG and LMB. About 4 years ago put in about 125 catfish.
Last year during the middle of the summer the water turned milk brown, this year just yesterday it turned back to brown again.
Thank you

It could be a type of planktonic/photoplankton bloom. Lets see if Bill Cody can impart some wisdom.

I have had catfish before, about 50 and for about 5 years then a winter freeze took them.
The only chemical that I use to control the algae is copper sulfate about 2 time a year and I have already treated the pond once this year.
Also I have a fountain for the summer and it converts to a deicer for the winter, it has a long tube to pull the water from the bottom for both summer and winter. This has work great for 7 years now.

How long after you treated the pond did it turn brown?

Colored water that is not a stain is almost always some sort of planktonic algae or dense zooplankton.

treated the pond two weeks ago

what could i use to kill the plankton?

While plankton is the building block of the food chain, you can have too much of a good thing. The plankton bloom is about right if you can see a white disc (top of a white 5 gallon bucket) that is 18"-36" below the surface in direct sun.

Less than that and it will allow light that might grow too many submerged plants. Less than that, and if all the algae dies off it might create a fish kill due to low O2 levels.

Green Clean Plus or Phycomiacin (sp?) will do it. Cutrine Plus is a copper based algacide that'd work too. Copper Sulphate will build up in the sediment over time. Trout don't do well with copper based algacides.