In article , Tim Nicholson
wrote:

I know this isn't strictly a gardening issue, but the group FAQ didn't
have any pointers, so I thought I'd give it a go here and see what
transpired.

I have a largish pond in the new garden, about 11' radius and approx
2' deep, apart from a ledge about 1' deep and wide round the
circumference. By my calculations this gives me an approximate volume
of 4,300 gals or 19,600 ltrs.

Like the rest of the garden, this pond has obviously been neglected
for at least 5 yrs before we got here, and although there are a number
of goldfish (around 35 we think) and a small number of larger fish
(possibly koi - I'm no expert), they're difficult to see because the
water is a particularlt turgid green colour. I suspect there's a fair
amount of debris on the bottom of the pond which won't be helping.
There's enough plant life round the edges to keep the toads and frogs
happy, and a few waterlilies which bloom quiute spectacularly in the
summer.

I think your best bet would be to install a filtration system -
solids-handling pump + mechanical/biological filter + UV clarifier
....or plant some shrubs - sunlight is the main promoter of algae and
you can dramatically improve the quality of your water just by
increasing the amount of shade.

Stan