Most of the time, watermeal is the last weed standing. And things that kill duckweed might have no effect on watermeal. Nuke the entire pond with Fluridone and start over. Put some new plants in to prevent nutrients from starting the entire process over. I don't think bumping up the concentration of active ingrediant is going to get done what you want. In fact, you may actually end up helping the watermeal by killing off everything it competes with.
