Turtlemtn, it's good to have you posting again after several months off!
Observations here: We bought our property 7 1/2 years ago and moved here 7 years ago. Our 1/4 acre pond lacked anything resembling vegetation, either weedy or algae. The water did not even have a greenish tint to it.
I guessed the soil lacked fertility. I now think that is erroneous. Now I believe the former owners and builders of the pond used lots of chemicals both in the pond and on the land to keep it "pristine", that is sterilized, nuked. There wasn't a "weed" on the property. Soon, "Aquacide" catalogs started showing up in the mail addressed to the former owners.
Two years went by before we notice a single plant in the pond. After 6 months each year of incoming irrigation water flushing through the BOW, I believe we may have rid the pond of most or all of the chemicals.
Months went by initially before I caught a fish back then. I found PondBoss after a couple of years and wow, did that open my world to management possibilities.
Now we have an overabundance of Elodea, quite a bit of FA through some of the summer, a little of APW and a grassy kind of growth that gets about 3 inches tall that is growing in the 3 foot deep areas.
Fish, once quite rare, are all over the place.
All this is to say that you certainly may have some residual effects from what was used in the pond before you arrived. Time and water changes may be required for your pond to recover if that is the case.
Tests for pesticide residues probably are not cheap. Maybe someone else here can offer remedial advice. Imagine some sort of carbon filtering system-biochar, partially burned logs placed in then later removed from the BOW, pumping some water through a substrate with fungal mycelia?
Good luck, and please post your thoughts and trials.
Roger