You're dealing with close to a 50 year old pond that has not had aeration until recently. Working with old ponds is definitely a challenge, a big challenge. Aeration tends to slow the aging process. The pond each year has been accumulating nutrients, growing more and more plants that have been dying and recycling lots of the available accumulating nutrients and most of the nutrients are dead organics bound in the bottom anaerobic muck that creates the sloppy black sludge in the bottom.

Duckweed and water meal are nature's indicator plants of an over-enriched condition (hypereutrophic) that is a normal process or status for an aged or old 'dying' pond. I often see duckweed in drainage ditches that receive enriched septic tank outflows. When a pond gets duckweed the pond is in aging trouble due to over enriched conditions and lack of other competing aquatic plants.
Lack of other nutrient using aquatic plants allows duckweed to grow unchecked. You are experiencing nature's way of filling in that pond. Until all that muck and bottom slop is removed, containing now all that huge nutrient enrichment "sink" ,,,, the nutrients contained in the system will demand some type of plant to grow profusely and respond accordingly to utilize that over enriched condition. Duckweed loves hypereutrophic with no other plant competitors. The more unused nutrients that are present results in more plants that grow.

To keep that profuse plant growth from happening one will constantly need to "chemicalize" that pond. Expect to chemicalize annually. Pond chemicals are not cheap. MuckAway, GoClear and similar products are not guaranteed and there is a reason why. Even if one could find a 'magic" bacterium to consume the nutrients, the biomass of bacteria needed to do all this would probably equal the biomass of duckweed. Then when all that bacteria cloudy 'soup' eventually dies, what happens to them? Nature says they will run out of food (nutrients), die in mass, decompose, and recycle the nutrients back into the system. What was accomplished? Then it becomes another do-over. Accumulated nutrients in the basin from the aging process do not go away. They eventually just get recycled and or bound up in black bottom slop. Non-flow through ponds are just BIG accumulator toilets with no flusher. Eventually the accumulations fill in the pond to become shallower and shallower moving toward swamps / wetlands, then eventually dry land (succession). Without proper pond management from day one, ponds age prematurely. You are experiencing first hand the pond aging process. People cannot stop the pond aging process, maybe just slow the process. Annual inputs determine how fast the pond ages.


Long term capital plan would be to drain and rebuilt the pond even if it has to be a smaller pond. If this is not done then expect reoccurrence of the recent disappointing conditions caused by excessive nutrient accumulations. Feeding pellets and growing more fish than the pond can support naturally speeds up the annual nutrient accumulation or build-up. Be aware feeding fish adds nutrients to the system.
Another option is to let nature continue to convert this pond toward the shallower wetland swampy conditions, deal with it as best you can and build a new pond and manage it properly from the beginning.