Thanks for the great info!
It's funny, but the progression of the algae/duckweed was the other way around. We had tons of the filamentous algae, but after we had the pond mucked out, the duckweed took over.
I will wait until the pond clears to add the aeration.
Most of the agricultural activity in the watershed is our own animals (Pigs, chickens, and goats.) I suspect that the nutrient levels are high and due to runoff from our pastures more than from the spring. I'd be curious to get both the pond water itself tested and the spring water to test my theories.
I did the settling test with a sample after the pond was mucked out but not after it was enlarged, so it's probably worth trying again. I also did a test with gypsum and chose not to use it because it didn't affect the settling/clearing rate much.
Among other things, we use our pond to irrigate our 1 acre blueberry field. Blueberries require an acidic pH, so adding lime to the water isn't an option for us. Gypsum would be an alternative, but might not have much benefit as per the test I did above.
Thanks again.