"Harry Ziman" hziman at clara dot co dot uk 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.

"Tumbleweed" wrote in message
Last year i installed a system with UV plus filters in it. It worked very
well, crystal clear water but hassle to clean out the filters.
This spring, after starting it up again, I hadnt got round to putting new
filter sponges in and I knew it would clog those up very quickly, so I
thought I'd leave it running filter-less (essentially just UV-treating the
water) until perhaps some of the algae had been killed. The water is now
crystal clear again, no filters yet.

Alex Woodward writes

The UV will help to keep the pond water clear, but it will do absolutely
nothing in ensuring that the water is healthy for fish or the majority of
wildlife to live in.

I don't think its straightforward.

I built my pond about 6 years ago, 44ft x 15-18ft x 3-4-5ft deep, and I
have had no real problems with plenty of fish breeding in it and lots of
wild life. IME the water quality varies with the location, natural
shading and surrounding trees and shrubs, the seasons and the sunlight
and temperature and the availability of nutrients and oxygen, and these
balances change continuously with the weather and as the plants grow and
use more or less. My water is generally clear and I can see fish down
through probably a couple of feet, but it varies enormously.

Although I have a large external filter above the bank (hidden by a
clump of bamboo) there are times when the water is cloudy - when the
algae are growing rapidly due to temperature and sunlight and nutrients.
But a couple of days later and it can be crystal clear. The filter (I
believe) enables bacteria to act on the filtered material. The pump
needs to be strong enough to lift the right volume of water up to the
top of the filter box, which in my case is about 8-10 ft. The filtering
depends on sizing the capacity of the pump v the volume in your pond,
and what you want to keep in it. It should be able to filter the whole
pond's worth every few hours for certain fish, or less so for plants.

Periodically I get blanket weed, and yes the barley straw (it must be
barley) does work - though I think it takes a year or so to build up
full effects. I also have used a pink crystal called 'Pond Balance'
which also works (but is expensive for my size of pond).

Remember your plants will grow, and surprisingly quickly! I removed 5 cu
m from the surface of mine this year, which is a huge amount. All you
can hope to do is 'balance' what is going on over the weeks and months,
I don't think it will ever be static and stay as ideal as you wish. But
that is the fun of it, the variety, for me. Every day is different!
--
David