How I got rid of green water in my pond...

....Hi folks,

As a long-time (ever since it first went green after I filed the pond up
2 years ago) sufferer of green water I have finally cracked it, and am
now looking through crystal clear water to the bottom of my pond, and
all the happy fish inbetween.

OK, so to set the scene, I've always had a big filter. This is a 2000
gallon pond with a LARGE settling tank (very big water butt) and a large
biological filter (large wheelie bin) with about 40kg of porous ceramic
and loads of layers of foam - closing to two layers of fine matt at the
top. I had run a 10w UV for about 6 months but that did nothing.

The water was traditionally pumped back from the filter to the pond by a
Grundfos central heating pump. Water visibility hovered around 6" most
of the time.

When I'd switched the pump off over the winter (probably won't do that
again - I'll just use a heater to keep it about 4 degrees) the
visibility slumped to about 3" and one big Orfe died off - maybe due to
lack of oxygen. Algae likes winter and dark evenings, mine did very well
indeed.

....so I bought a nice big 45w air pump and 2 12" air stones - which
proceeded to create mountains (icebergs) of foam and cleared the water
to about 18" visibility. This was a very good start and you could see
daily improvements, but the water never cleared further. I then
installed an extra pump, so I had 2 pumps pumping through 22mm back to
the pond (one via a big trickle filter) each turned down in speed.

The water clarity stayed about the same, foam was sparse on the surface
but still occasionally visible in the mornings.

An air pump in the pond then certainly seems to be a huge help, the fish
like the bubbles, water circulation is excellent and the green is cut
down quite a bit - but still the water was greeny, even after 3 weeks.
When the filter clogged the washing effort was like washing out thick
green paint!!

The next move was to properly install the retro-bottom drain. The usual
outlet is a 4" pipe about 12" below the surface, and I have connected
this to about 12" from the main bottom of the pond once before - but not
right resting on the bottom - plus I wanted to get rid of the foam.

So I got another 4" branch and hooked the system onto a regular
(narrower) drain pipe, going to a drain-pipe collector attached to a
grill - giving about 12" by 8" area resting on the bottom. This now
allowed water from the very lowest point in the pond to travel up the
tube, and the branch was arranged so together with a grill the surface
water could drain into the (now at about 1-2" lower pressure) branch too.

I set this up and went off on holiday for a week and when I got back the
pond was totally and completely clear. Like I could now see every detail
4' down on the bottom, and every single fish. Even the dark ones!!

So having struggled with green water for 2 years, my own 'patent' remedy
for this was:
Use a pond air pump and air stones
Collect water from the very lowest point of the pond
Skim water from the surface of the pond

and QED - no green water!!

I hope this helps anyone out there with a puzzling green water problem,
it's much nicer to look at now )

Graham