Copper Sulfate (also called Complexed Copper, Chelated Copper)
Copper sulfate isn’t really an herbicide, copper is more of an algicide, (but can also work on hydrilla).
Copper sulfate and the newer versions of complexed copper are extremely effective on floating algae, and chara, which looks like a brushy lake weed, but it’s actually an algae. You probably have chara blanketing you lake bottom.
Copper has a long history. Romans used it to prevent algae growth in their Roman baths. Farmers use it to keep algae from growing in ponds their livestock drink from. On land, copper kills bacteria, viruses and fungi that come in contact with it, by (in effect) rapid dehydration.
We don’t sell copper sulfate, but we recommend it regularly, most often to people who have chara with one important caveat: use it early in the season, before August.
As summer progresses, chara forms a sort of armor on itself, so by August it’s very hard to kill with copper sulfate or any chemicals.
That said, LakeMat Pros and MuckMat Pros will kill chara anytime.
Get copper sulfate at local feed stores, Tractor Supply, even on Amazon.
LakeMat Pros are for lake weeds only.
MuckMat Pros are for both lake weeds and a firmer lake bottom.