Dust and dirt blow into the pond.
Just a fact of pond life. A natural pond, over
its life cycle is meant to fill in, so your pond
is just doing what comes naturally.

A mechanical filter consists of some kind of
substance to catch all the mulm. A good way
to include one in your pond is to build a waterfall
and set the mechanical (usually comes in combination
with a bio filter - a second set of screens or bio balls
for the growth of benefical bacteria to convert fishy
ammonia) - okay, where was I... set the filter
up behind the water fall. Water goes from the pump
to the filter and then out to the waterfall.

A skimmer can also help keep the water clean
and a bottom drain, even retro-fitted helps with
cleaning the pond.

That said... which seems like a lot of work ;-)
that said, we have a big stock tank at the back
of our waterfall (no skimmer or bottom drain).
The water goes in the bottom and out the top.
We have used black vinyl screening to catch mulm.
Right now we put water hyacinth in the top. The
roots catch the mulm, the plant filters the water.

And we attempt to clean out the bottom of the
pond each spring.

Hope that gives you some ideas. But the 'dust' is
a natural part of the pond.

good luck!

kathy :-)
www.blogfromthebog.com
this week - turtles!