The message
from Dave contains these words:

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.

In a typical sand filter the grains are coated by an alga which removes
impurities (nitrates, etc) and noshes bacteria.

The algae you don't want are filtered out mechanically, and die and
decompose, and the by-products of this process are then taken out by the
alga in the sand.

You don't have to prime the filter, the alga in question is naturally
present in the water. It will take about a fortnight for it to be on
song.

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!

Yup - I was going to suggest barley straw, but I see you have already.
If I were doing it and had room, I'd consider passing the water through
a small reedbed, then the straw, and into a sand filter.

Seriously, whatever the quality of the water which went in, you could
safely drink the water which came out.

If anyone is interested, I'll make a set of instructions and a diagram
and put the result up as an HTML page on the wibble.

--
Rusty
Open the creaking gate to make a horrid.squeak, then lower the foobar.
http://www.users.zetnet.co.uk/hi-fi/