We have a 200 gallon shallow bio pond with rocks on the bottom (no fish)
the ponds water flows through this bio pond, (via underground tubing)
getting scrubbed by the rocks and vegetation in its path before traveling
back and re-entering the main bare bottom gravity fed BD pond.
We also have a 4 X 5 foot rocked shallow beach section that we hose about
every 2 weeks...A tremendous amount of muck is loosened and expelled
from these rocks and it travels down and out through the BD. The rocks
are then clean and sparkly after the cleaning. ( I know I showed you guys the video
of the beach cleaning, and the clean rocks afterwards.)
Getting back to the bio pond, it is cleaned only once in early spring, it stays clean
for the rest of the season... remember there is no fish in the bio pond.
BTW...We do not use a UV light or chemicals, and our pond is always clean
and balanced.
Having said all that, I believe you can maintain a clean and healthy pond with rocks,
but it's a lot more work and a lot more expensive, if the pond is large.
I myself rather not spend endless hours vacuuming and emptying our pond yearly
for cleaning maintenance...
It takes the fun out of the hobby.