You can buy an entire box of bioballs at any petshop for about $5. Why
anyone would want to use them for a garden pond is a mystery to me. I've
even removed them from my marine aquarium because they are a source of
unwanted nitrates. Better to have a natural biofilter that will absorb
these nutrients and that can be harvested, thus elminating those nutrients
from the pond altogether.

Some readers will get confused by this paragraph. You don't mean just any
"natural biofilter," but rather something /specifically/ like a veggie filter.

In marine aquaria this is achieved with an "algal turf scrubber" or similar
technology, as well as heavily planted refugiums (e.g., thalassia sea grass
is particularly beneficial for reef aquariums, because not only does it help
remove nitrates, but also provides an area for the breeding of zooplankton).

C//