Green water couldn't be easier to cure. Change 80% of the water two days in
a row while you're diatom filtering the tank. Vacuum the gravel THOROUGHLY;
expect to find a lot of crud there and tons of green water coming out of the
gravel - this is why it reoccurs.

In a tank with healthy plant growth from good light and fertilization,
that is kept clean, you will never see green water.

Show me a tank with green water and I'll show you where it's dirty,
which is why the water turned green.

The green water represents free ammonia that shouldn't have been in your
tank. Even if you were to kill it all with say, a UV sterilizer it'll
now rot and go back to ammonia only to repeat this cycles as ammonia
is what causes it. You have to remove the source of the problem and the
problem itself. You might be surprised at how much crud you can remove
from a green water tank. On about the fifth bucket of dark brown
water you start to get some idea of just why this tank turned green.

A cheap trick to keep is clear is to throw in a handful of water
sprite or other agressive floating plant.

Long term, water goes green because of underfertilization and
lack of frequent partial water changes. That is, the plants
are not growing quickly enough to consume the ammonia the tank
is producing.

Ambulia planted int he gravel seems to work as well as floating
water sprite - any FAST growing plant will work. Note this
does not mean "any stem plant". While Ludwigia repens would
also be a good choice, Ludwigia, say, gladulosus or arcuata
would not be.

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