White, Green and Yellow Cloudy Aquarium Water

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Some of the more common causes for cloudy aquarium water are bacterial blooms, algae blooms, high levels of dissolved organic carbon and/or the use of new driftwood.

White Cloudy Aquarium Water
If you have white cloudy water or gray cloudy water in your fish tank and you have just set it up, it could be the dust from the gravel or sand. Be sure to thoroughly rinse your substrate before adding it to your aquarium. This can be difficult to do with sand but the dust particles should settle in a day or two.
If your tank has been setup for a short time this may be the result of a bacterial bloom that happens in a newly setup tank or when too many tropical fish were added too soon. This situation will correct itself when a sufficient amount of bacteria establishes on your biological filter. See the Nitrogen Cycle (http://www.fishlore.com/NitrogenCycle.htm) for more information on this process. You can help keep your tropical fish from getting stressed by performing 25% water changes daily and feeding them less until the nitrogen cycle has finished.

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Green Cloudy Aquarium Water
Usually the result of an algae bloom. The green water will not harm your fish but it is not the most pleasant thing to look at. This happens because of the amount of nutrients and the amount of light entering the aquarium. Your tank water is nutrient rich, which may mean that you're feeding too much, your tank may be overstocked or you're not doing enough water changes or a combination of all the above. Avoid placing your aquarium where it could receive direct sunlight. Direct sunlight will cause not only algae problems but temperature fluctations as well.
To correct this problem, perform 25 percent water changes daily, rinse out or replace the filter media to reduce the amount of nitrates, feed your fish a little less and leave your tank light (http://www.fishlore.com/aquarium_light.htm) off for a few days or until the water clears up. If you do practice good aquarium maintenance and still have algae problems, look into testing your tap water. You may be introducing nitrates and possibly phosphates into your tank via the water changes and not even know it. Also look into keeping live aquarium plants which will compete with the algae for available nutrients.
Pond keepers sometimes use a UV Sterilizer (http://www.fishlore.com/uv_sterilizer.htm) on their outdoor ponds to help control algae growths. While we agree that these sterilizers can have a positive effect on the algae growth in your tank, there are better, less expensive methods to use for controlling algae in the aquarium and we recommend you try some of the ideas presented above. If that doesn't work, as a last resort, you could use Aquarium Pharmaceuticals Algae Destroyer. The algae destroyer should clear up the algae problem quickly. Please use as directed on the bottle. This is just a quick fix and we recommend that you figure out what's causing the algae problem in the first place.

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Yellow Cloudy Water
Yellow aquarium water can be the result of several factors. Fish waste, driftwood, decaying plant matter and other dissolved organic carbons (DOC). This often happens in tanks that have new driftwood and it is because of the tannins in the wood being released into the water. This should subside after the driftwood has been in the aquarium for a couple of months. To best way to fix the yellow coloration of your tank water is to perform more frequent partial water changes along with the use of activated carbon. Activated carbon has many uses in an aquarium filter including removing small amounts of DOC, removing water smells and making the water look cleaner or polished. Try to change out the carbon on a regular basis, at least every couple of months if not sooner.