I'm sceptical that the imaged specimen is
Hydrilla . It could easily be
Egeria densa or
Elodea canadensis .
All three share close similarities and are often confused; yet the accurate assessment of identity is very important when determining an appropriate control-measure - especially herbicide-selections.
I'm curious if anybody can explain what keys shown in the OP's photo allow an absolute identification as Hydrilla.
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BTW: John, your comments sure read like a recommendation to me.
Acute mortality to fish isn't the only adverse affect that can accompany "off-labeled" uses of certain herbicides (or their surfactant components) in aquatic habitits.
IF you insist on using off-labeled products in your pond, that's your business - although I certainly can't understand the rationale for doing so; but posting off-labeled "experiences" on a public forum is pushing the envelope.
What happens if a reader acts on your suggestive prose but encounters far different results?
I'm sure the product's manufacturer will be thankful that they're not liable for such off-labeled uses.