Surface or subsurface flumioazin for watermeal? I've done two surface treatments and I'm wondering if subsurface would be better.

Background:
I did a surface treatment with flumioazin in late April with a low dose and treated just the areas where the watermeal was concentrated by the wind. It knocked the watermeal back but within a month it was back to ~50-75%% where it was before the first treatment.

After the recommended 28 day waiting period between treatments, I did a second suface treatment at the full recommended dose and followed-up a couple days later with some spot treatments. Will be evaluating results but subsurface seems easier to apply and may be more effective on 100% of the pond.

In both cases I treated before the quantity of watermeal was big enough to trigger a fish kill