Assuming 2.5 ac-ft was treated with 4-oz of a 4#a.i./gal fluridone ("Sonar AS"), the fluridone concentration should've reached somewhere around 18ppb, which is a bit low for many lily species but should be quite adequate for EWM - if sustained.
At 1-month post treatment, expect to see the initial discoloration on the EWM's terminal-growth to become less noticeable since the terminal growth (meristematic tissue) detaches (abscises) from the remaining stem, leaving a "snipped-nub" at the point of abscission.
At that juncture, the remaining biomass of EWM will remain static for some time (2-4 weeks) with further visual responses occurring at glacial speed.
Eventually, and often in a matter of mere days, the plant biomass will quickly (relatively) drop to the bottom - or possibly remain suspended due to intermingled accumulations of filamentous algae (which provides buoyancy).
In your situation, I would be concerned if:
1) no initial discoloration was observed in the EWM's meristematic tissues.
2) abscission-nubs are not currently observed in place of active terminal-growth.
Inspect your EWM for the above conditions and report back.
