I have this problem too.
Regularly having to take out heavy duckweed growth from the surface of my 'tadpole nursery'
(6 x 10' x 15" deep = back end filter settlement lagoon for a much larger fish pond).
If I leave it too long, it gets too dark under the water,
& new soft algae (tad food) doesn't grow enough, & they risk starving.
Trick is to save all those soft squigglies from doom as you remove the duckweed.
Best method I've found is to (double) hand scoop the worst away,
& drop the handfuls of duckweed into a 50 litre 'fun tub' half+ full of water.
Then most of the tadpoles will have vacated the near-surface of the pond
& you can net the remainder of the duckweed (into the tub).
As long as you shallow scoop.
When you got all the weed (+some tads) in the tub,
stir it around, which puts nearly all of the tads to the bottom,
and shortly after you can hand-scoop/net the weed from the tub into the green bin.
Actually the best way is to use two tubs (first tub is cleared bit at a time into the 2nd),
so you can 'dilute' the duckweed & make sure the tads are at the bottom of the tub.
When nearly-all/most of the weed is removed from the tub,
then pour/bail the tub back into the pond,
along tads (& all the other squiggly critters).
Only takes say an hour or so, well worth the effort...
fingers crossed.
