Like many people here our pond developed it's usual pea green algae
bloom this spring. On top of that the fish had been mostly unseen for
about 18 months after a plant emptied it's soil into the pond and turned
it murky.

So, after reading all the useful replies here, I decided to tackle the
problem. The methods that I chose were UV clarifier and filter wool.
The UV device is an 18 watt basic unit off of eBay, it cost about £45
with postage and packing. We plumbed the UV clarifying in between the
submerged pump and filter tank. The filter wool is 12mm thick, 1m wide
and came in a roll of 10m. We layered that two ply on top of the filter
media as a final filtration step before water exited into a small veggie
filter. Without the wool I don't think my Skipy style filter would have
caught the flocculated algae cells.

The result -
After one week the water was looking less murky, but still pea green.
After two weeks the water was 'transparent' but green.
After three weeks the water was crystal clear.

I suspect that the dissolved solids were rendering the UV clarifier
almost useless to begin with. Once the water was clear and 'transparent'
the UV could travel further and do more work on the algae.

The filter wool clogs up quickly, it will change from white to brown in
a few hours. We cut it in to squares that fit the filter tank and used
either 2 or 3 layers. They needed changing or cleaning every 4 or 5 days
while the algae was being removed. Now that the pond is clear they last
a week. It's wonderful being able to see the fish again, so I highly
recommend tackling the problem seriously to solve it in time for summer.

Thanks again for the r.p.m wisdom.

--
DavidM
www.djmorgan.org.uk