@androx and charlie- i would highly suggest you using uv lights to greatly eliminate green water and algae, the initial investment is kinda expensive but the maintenance is not that costly, you should only change the CFL or flourescent lamp. unless naguba jud imu UV machine.
regarding sa pond, i assume that your filter is also outdoor nga chamber type diba? what we did before sa akong friend is we bougth the uv light lang nya e pa siga lng sa one of the chambers, so basically mura siya aquarium light nga nagpahayag sa sa 1 of the chambers sa pond. kamu lng assemble sa lughting case and fixture for the uv light. just be careful lang na ayaw e direct contact sa mata, coz remember wa bya ni case unlike sa mga branded na uv light fixture.
basta main point is buhat ka ug lighting fixture nya imu suga is uv light lng, e mount ug wood or anything nya e butang sa taas sa usa ka chamber sa imu filter. tabuni lang ug wood or anything not transparent.
HTH.