why does the water in the fish tank turn green when there is a filter and a light?

Way too much light, or maybe light of the wrong spectrum. If you are using flouro lights and the bulbs are starting to get a bit old, you might find that the degraded light quality from the flouro bulbs are causing at least part of the problem.

Change your flouro bulbs to a proper spectrum for your tank, and reduce / eliminate the amount of daylight and you will get rid of the algae

The filter doesn't help with green water.

The green is free floating algae living in the water, it's too small for a normal filter to remove.

Algae needs two things to grow, light and nutrients.

Light - either you are leaving the light on too long, or the tank is close to a window and getting too much light that way.

Nutrients.- Keep up the water changes to reduce the amount of nutrients in the water.

Some algae is normal in any tank. but if the water is turning green, then you have too much light.

Ian

There 2 main causes for green algae.

1. Light:

This can be either direct sunlight or the amount of light the tan is getting. If your tank is not in direct sunlight, reduce the time the light are turned on.

2. Nutrients

High nitrates is another reason for thriving green algae. If you have more than 30ppm, due smaller frequent water changes to reduce the amount of nitrates.

algae thrives in sunlight, i had that problem a while ago when i first got my fish, so I put my sketch book behind the tank which prevented most of the light getting in so it now takes a long while for algae to grow.

Try moving ur tank to a darker area or covering one side.

hope it helps :)

filter doesn't have anything to do with this, maybe your tank gets direct hit sunlight, because sunlight is the cause to algae. if so then relocate your aquarium if u wish. hope i helped :)