Chronic Green Water Problem

Charlie Farley wrote:
My 100L tank has only one fantail goldfish (about 4"). I replace 30L
of water every 2 weeks and have 2 x Fluval plus 3 internal filters
with a strong air bubble feed.

For about 6 months the tank's had green water. I use King British
Green water control (synthetic organic polymer) - the green always
comes back and this dose my fish seems to be unable to come up for
his food.
Is this stuff safe for long term use?

Any advice - much appreciated

Thanks Geezer & Sophie - I'll try 10L (fresh water) a day for a few weeks
and check out my nitrate level.

I visit this NG about once a year and always get solid friendly help -
thanks so much.

Charlie

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Charlie Farley