We have a "beach" about 50 x 150 feet. Comprised of black matting then about 6 to 8 inches of small, smooth rocks / pebbles (about 1 centimeter in size) on top. Some have washed into lake and that is now a prime bluegill spawning area. Weeds have gotten started on the beach. I don't have time to pull them. Any recommendations for chemicals to use to control them without harming fish?

It depends on the type of "weeds". Some "weeds' are actually algae (Chara).

If you can ID the weed I'm sure we can give you options.

As for not harming the fish, that really is up to the person applying the chemical. Don't read the label for proper application? Possible fish kill. All the herbicides/algaecides that people in the know here on the forum recommend are approved for use in ponds and safe for the fish.