11-19-2012, 01:39 PM
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Bedfore spending money on UV, let's ID the cause and fix that.
Green water is multicellular algae in proliferation to the point that the water clouds green in colour. [What follows is taken from a site on algae, written by Dusko Bojic, to give credit.] The situation that causes GW (Green Water) is usually a combination of high nitrates, phosphates, and mixed in some ammonia/ammonium. Substrate disturbance is usually the culprit. What happens is the algae (GW form) will flourish off of the ammonia/ammonium and phosphate, remembering that algae can consume phosphate easier than plants because of their thin cell walls, the algae uses up the ammonia/ammonium and phosphate, but it doesn't go away...because algae can quickly switch which nutrient it scavenges...it moves to nitrates. So you can see why water changes will not rid a tank of GW. Nutrients can be reduced very low in GW and fairly quickly by the GW algaes, but they can scavenge other nutrients...iron and trace elements. So, it's very common for the GW to solve the situation that causes it to begin with, but that won't eliminate the GW, for the reasons I've allude to. [end of citation.]
Reduce feeding, do good substrate cleaning (open areas) during weekly water changes of 50% of the tank volume. Tank light and ambient room light is also critical. If there are live plants, the light must be sufficient for the plants (intensity) and no more, and then on for no longer than all nutrients are available (duration). A complete blackout (no light, and wrap the outside of the tank in dark paper/towels, etc, to keep it totally dark for 4 days) should deal with this, provided the cause is cleaned up first. Otherwise the green water may return once light is restored.
Do not use any chemical sold for this problem; all of them will cause trouble for fish, and might kill live plants.