Good job treating the symptoms, but missing the problem. The problem is that your pond is eutrophic, or over fertile. The weeds in the water are what are stripping the excessive nutrients (nitrogen, phosphorus). The water plants were what were putting oxygen back into the water. Without them, oxygen is stripped by aerobic decomposition until the pond becomes hypoxic, and anaerobic decomp takes over. (Stinky stinky!)
Numerous water treatment plants are now using the duckweed as the nitrogen strippers in order to release clean water back into the environment. The duckweed was helping. By skimming it almost daily and dumping the plants in a compost heap will help remove those nutrients from the water. Same with aggressive harvest of any water weeds. Think of the water borne plants as nitrogen and phosphorus sponges.
By introducing plant eating fish, you simply add another step in a closed circle, i.e. the fish eat the weeds that are feeding on the nutrients and pooping out more nutrients for more weeds.
There are two short term ways of fixing the pond; A) the old adage, "the solution to pollution is dilution", but maybe you don't have access to a lot of fresh water to add to the pond, or B) carbon sequestration de-nitrification, let the plants do the work and remove the plants on a regular basis. The only long term solution is to reduce ag runoff, something not likely to happen in Iowa until the collapse of civilization as we know it.
