Any thoughts? I'm not going to be putting any herbicides in the tank
and I'm dubious of anything store-bought that will magically fix the
problem (I work in a LFS.. I've yet to be amazed by anything off the
shelf, except a very old bottle of Aqua Safe that smelled surprisingly
like rotten eggs).

Look back on Green water on this list and also the APD etc. Green
water will not be cured through nutrients/snake oils etc. Diatom
filtration, UV, Micron filtration(eg Hagen quik filters are 5
microns), Daphnia, blackouts(not very effective in most cases) all
work. UV./Diatoms are the best IMO/IME.

Once you remove it, let the UV/Diatom etc run another 2-3 days.

Large distubances combined with poor plant growth/low CO2/high light,
fish loads etc, will help induce GW.

Once you get rid of it, it will not likely come back again.
But once induced, it is very difficult to get rid of with mechanically
removing it or UV.

You need to work on providing good CO2, macro and traces elements to
your tank if you want the plants to grow.

Generally you want 5ppm or more NO3, .5ppm or more PO5, 10-20K+, GH of
3-5, traces dosed 3-4x a week etc.

So a routine for your tank:
1/2 teaspoon of KNO3 3x a week
1/8th teaspoon of KH2PO4 2-3x a week
Traces 15mls 3x a week

50% weekly water change.

The dosing will keep anything from running out while the weekly water
changes will prevent any build up.

In that manner you can easily control the parameters with a testing at
all, except for CO2.

Regards,
Tom Barr