In article , Harry Ziman
wrote:

We are about to install a large pond in our garden. We don't intend to keep
fish in it, but do want to keep the water clear.

The advice we have received is that we need a UV clarifier to prevent algae
formation but, as there will be no fish, there is no need for a filter to go
with it. The algae will clump together and settle to the bottom.

However, looking here, it seems to suggest that a filter is needed - does
anyone have any experience and if so can you let us know. I don't want to
make an expensive mistake and install the wrong system.

Fish are only one cause of dirty water. Other contributors include
wildlife, plants (rotting organic matter), grass clippings, silt, etc.
The first prophylactic measure must be to site the pond in the right
place - preferaably sheltered so that debris doesn't get blown into it
and definitely in partial shade since sunlight is the main catalyst for
the growth of algae. You should also plant some oxygenating plants and
possibly also introduce oxygen to the water by installing a fountain
and/or waterfall to break up the surface.

Then you should install both a UVC and a filtration system (or a
combination unit). And since filters are sited outside the pond, you
must choose between a box-type filter which you will have to hide
somewhere or a pressurised filter which you can mostly bury out of
sight beside the pond. Most filters incorporate both mechanical
filtration (via layers of foam) and biological filtration (a colony of
beneficial bacteria which attacks the harmful bacteria).

Since you are understandably worried about wasting money, you should
take advice from one of the big manufacturers such as Hozelock Cyprio -
http://www.hozelock.com/aquatics/I029.asp?region=ROW - who will give
you a money-back guarantee if their recommended system fails to give
you clear water. I recommend their Helpline - 01844 292002 - and
suggest that you give them a call, armed with the dimensions of your
pond.