There's probably still time to treat with fluridone, but with an elevated risk of other consequences DEPENDING on the weed-density and other site conditions. Also, achieving "control" with fluridone will now take much longer in comparison to early-season treatments. Lastly, higher-concentrations & sustained exposure-times are much greater factors for fluridone performance as targeted weeds gain maturity.
There are other available treatments (chemistries) which enable localized (partial-area) control that are worth considering at this point in the season. More labor is generally involved in such treatment, and probably at a relatively higher cost "per treated-acre" compared to a whole-pond fluridone treatment, yet partial-pond treatments will generally provide much quicker relief (within targeted areas) and avoid a pond-wide crash of treated vegetation.

Kelly Duffie
Cypress, TX