If you do try one of the products that are supposed to clear the water, or if you have already without success, then check out the other suggestions below. Hopefully one of them will work for you. Though, I, as well as many other people have found that just plain letting the water sit helps the most.

CAUSE:  One thing that can cause cloudy or green water is over feeding. The feed dissolves and mixes with the water, much like what happens when you are out wading in the water in a lake. The mud gets stirred up and clouds the water. The same thing applies to your aquarium. Decomposing food mixes with the water. The agitation in the water from your filtering system keeps the dissolved food stirring/mixing around in the water.

SOLUTION:  Shut your filters off for an hour. This will allow the food particles to settle on the bottom of your tank. Do a real thorough gravel cleaning. Clean up all the excess food particles.

PREVENTION:  Do not feed so much that the food has a chance to fall to the bottom of the tank. It is far better to feed small amounts of food 2 or more times a day than it is to feed a whole lot at one time. Don't worry, your fish will not starve to death. Fish can actually go a couple of weeks without food, though I would NEVER recommend that. It would not be good for your fish. Another thing that would greatly help alleviate this type of problem would be to get a bottom feeder type of fish for your aquarium. I would suggest Catfish, Plecos, Loaches, etc. All of these types of fish are great for that sort of thing. They can burrow down into the gravel and clean up excess food that you can not even see. They have the added bonus of being very fun fish to watch.

CAUSE:  Another thing that can cause cloudy or green water is an excess build up of waste matter. This would depend on what type of filtering system you are using and how long your tank has been up. Undergravel filters build up waste material underneath them. Eventually the area underneath them gets full. An undergravel filter works by sucking water down under the filter and then back up through the tubes. Because of this, when the area underneath the UGF gets full, there is no where for the waste to go, but back up the tubes and out into your tank. Thus causing the same effect as I described above. The length of time it takes for this to happen would depend on the number of fish as well as the amount of wasted food.

SOLUTION:  Do a total thorough gravel cleaning. Put the tube down clear to the undergravel filter. Hold the tube there until you see clear water come out of it. Repeat this over the entire bottom surface of your tank. If you have a small tank, you will have to do this at more than one time since you will quickly run out of water before you get the entire tank clean. Another way to do it would be to refill the tank and start where you left off until you run out of water again. You would need to refill the tank several times (depending on how dirty it is) before you would get the entire tank clean. Once you have nothing but clear water coming up the tube then your tank will be clean. NOTE:  Your tank will have to go through the cycling process all over again. You can speed this process up by using StressZyme. Use the dosage on the bottle for starting a new tank.

StressZyme is put out by Aquarium Pharmaceuticals. This product adds live bacteria to aid in the development and maintenance of your biological filter. I am totally sold on this product. In my first 2 tanks I used nothing. What a mistake. I had nothing but problems getting my tank to cycle. When we purchased our first 50 gallon tank the man at the store suggested using StressZyme. The tank was fully cycled in no time at all. I didn't have to contend with cloudy/green water or high ammonia content. I use it every time I set up a new tank and after each water change. You always take some of the natural biological filter out during tank cleaning and this replaces what is lost in the process. After a water change add enough to bring it back up to full strength. IE: If you take out 5 gallons, add the amount of StressZyme required for 5 gallons of water.

If you have the added luxury of having more than one tank as I do, you can move your fish to another tank and take everything out of the dirty tank and clean it. This takes a lot of work but is well worth it in the end.

If you have a canister filter or a Bio-Wheel type of filter make sure that you filtering media is clean. I use Bio-Wheels and Magnum Canister filters. If they get full there is no where for the waste matter to go but back in the water. Make sure you clean or replace them every time you clean your tank. This will aid greatly in keeping the tank clean as well.

PREVENTION:  When you clean your tank, clean only 1/4 of it thoroughly at a time. Go clear down to the undergravel filter. Leave the tube where it is until you see clear water come out of it. The next time you clean your tank, do the next 1/4 the same way. This way after 4 cleanings you will have your tank cleaned out. By doing only 1/4 of your tank each time you clean you will not disrupt your biological filter much at all. I have found this to work extremely well. Only remove 25-30% of your water at a time. Be sure and add StressZyme or some other comparable product after the cleaning. This will aid in replacing the bacteria that you removed by the cleaning.

CAUSE:  Your undergravel filter may not be placed correctly. We had one tank that had a real problem with cloudy/green water even though we were doing everything right as above. Finally one day while talking about it we remembered that when we set the tank up we neglected to place the UGF plates right up against each other. This did not allow the filter to function properly. If your plates are not in there tight up against each other your filter will not work right.

SOLUTION:  Remove as much water out of your tank as you can safely remove. Leave just enough for your fish to be covered. (If you have a second tank, put your fish in there.) Remove the plates and push all your gravel to one side. Replace the first plate. Make sure you have no gravel or a spare fish or two under it. Move the gravel all over to that side. Replace your next plate. Make sure that it is tightly up against the first plate and lined up exactly, front to back, with the first plate. Cover the second plate with gravel to keep it in place. Repeat the process until you have the tank back together. NOTE:  This is pretty stressful to your fish so be sure and watch closely for signs of ICK/ICH, Parasites, or Fungus for the next week or so. Be sure and add StressZyme or some other comparable product after the cleaning. This will aid in replacing the bacteria that you removed by the cleaning. Use the dosage for setting up a new tank.

PREVENTION:  When you set up a tank, place your plates as tight together as you can. Make sure that they line up exactly from front to back. Do not punch out the holes if you are not going to be putting an air tube in them. Be careful that no gravel gets underneath the filter.

CAUSE:  Live plants can cause a real mess in your tank. Bits and pieces fall off and decompose, mucking up your tank. It acts pretty much the same way that excess food does.

SOLUTION:  Do a good thorough gravel cleaning as described above. Clip off any ends that look like they are dying. Remove any plants that are not fresh and healthy.

PREVENTION:  I personally don't favor live plants, though the fish, I'm sure, love them. When I first started in my fish hobby I bought live plants. I had nothing but problems with them. Bits and pieces fell off and went to the bottom of the tank and decomposed. Decomposing plant bits can cause a rise in ammonia as well as a messy tank. If you have to medicate the tank for any reason, often times it kills the plants. Plants can promote algae growth and rob your tank of oxygen. After about 6 months of having live plants I totally eliminated all of them and went to plastic plants. When I notice algae forming on them that is not being eaten by my algae eaters I pull them out and throw them in the dishwasher. They come out bright as new. I know that there are people out there with live plants and they do just fine. However, my personal experience has not been that way. I guess I don't have a green thumb. :) If you do decide to have or keep live plants keep a close eye on them. Remove any ends or whole plants that are turning brown or do not look fresh and healthy.