Could it be because it's at least 10 years old? LOL

Using it to spot treat clumps of some type of pondweed species that is interfering with seining. Looks like bushy pondweed.

Applied a second application to see if it has any effect 2 weeks after the first one.

I seem to remember it can kill fish if overdone, and the fish are supposed to avoid treated areas, but that doesn't seem to be the case. Some of the bluegill fingerlings still hang around the clumps of weeds.

If fish are hanging around immediately after a "herbicidal-rate" of Hydrothol 191 is applied, either:
1) the product has severely degraded with age (unlikely), or
2) your dosage (ppm concentration) is insufficient for weed control (very likely)

Effective spot-treatments with either form of endothall are tough to achieve due to diffusion from the target-area occurring too rapidly. The smaller the target-area, the higher the required ppm-concentration in order to maintain a "plant-lethal concentration" for the necessary amount of time - often 4+ hours.
I rarely recommend Aquathol or Hydrothol for "spot treatments" unless the site-parameters are perfect.

Thanks Kelly!

I had it laying around so thought I'd use it.

The Hydrothol 191 did end up doing the job.