We have never tried a totally fishless pond. I suspect it would have
fewer nutrients than our koi/goldie pond as it would have less
fishwaste.

You are surefly right that algae and especially blanketweed will grab
nutrients and thrive. It cannot compete with more complex plants, so
your plantings will get rid of it as they succeed. We get a good dose
of blanketweed each year, especially on the falls. Our koi view it as
a treat, so it is not in the main pond. We let it run where it does
not mess up the view. It hastens its own demise by grabbing
nutrients.

Keep us posted on your progress.

Jim