03-23-2010, 07:15 PM
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With no plants (the one fern won't cover this), there is nothing to use the light and nutrients that naturally occur except algae, which is a plant itself. "Green water" is unicellular algae that reproduce so rapidly the water clouds green. It is caused by excess nutrients and light. An algae eater, or any such fish, would not help, since they could not eat this minuscule algae.
With no plants and only fish, you could reduce the light down to one tube, and then reduce the illumination period each day.
On the nutrients, do you know the nitrate reading? I'm betting it is high.
Seconding Stephanie's comment, please do not use any algae removing products. They usually don't work, and they will not work on green water--but even if they did, they also harm the fish. They are chemicals. Bad for fish tanks.
The other problem you have is lack of water changes. In non-plant tanks (and planted means well-planted) water changes should always be every week, and major. I do 50% changes every week and I have planted tanks where this is far less of an issue. Fish produce waste of all sorts that accumulates, organics get broken down by bacteria both in the water and in the substrate, and without removing some of this it just multiplies. Please do weekly water changes of 40-50%; your fish will thank you.