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

--
Charlie Farley