Pond Boss Forums MANAGING AN EXISTING POND Evaluating and adjusting fish populations Bass Dying all of a sudden

"O2 levels are lower or at least the water has the ability to hold less of it as the water is the warmest in the summer."

You got it! The warmer the water the less O2 for the fish, even without a vegetation kill.

Much safer in the cool months. Chemicals most effective when plants are putting energy down in the roots to over winter & next springs growth (Fall).

You can play it safe by doing your pond in stages, so that everything is not all decomposing & using up O2 at the same time. Perhaps doing just some areas & leaving others natural & see if you have the desired effect your are looking for? 20% is often mentioned as a minimum goal for the surface area of a pond to be covered with weeds. An area for biodiversity of the pond.