2. Clean decor: Thoroughly clean any decor in your aquarium. Often times if you don't rinse out new aquarium decor like substrate it can leach debris and such into your aquarium water and cause cloudiness from free floating particles. New driftwood can also leech into your aquarium water (some people actually want this for certain aquarium setups) and in this case, water will generally get a tea color tint to it. If your water gets this tint and it's unwanted then you can try adding in activated carbon to the filter system. A lot of people will soak or boil driftwood as this helps to cut down on the leaching effects it leaves in the aquarium, and it may take numerous attempts to get completely get rid of this tinted water issue.
3. Water changes: Depending on how quickly your aquarium gets dirty from its inhabitants, it may be time to start doing some partial water changes with clean water, this is often the best way to remove cloudiness or discoloring from aquarium water. Adding in some aquarium salt may help to clean 'established' aquarium water, which may help remove cloudiness. Therefore, I would suggest not using aquarium salt in an unestablished aquarium.
4. Plant waste: If you have live plants in your aquarium you may find they decompose a bit, especially if they don't have the proper habitat needed for proper growth. Decaying plant matter can also cause water to become dirty and cloud. Also be sure you don't leave your aquarium light/s on for extended periods of time as your cloudy water may be caused by algae bloom (which is often green in color).
5. Chemicals: Some water chemicals may cause water to get cloudy, be sure you're using the proper amounts and perhaps find different products to use if this is the issue. There are also some chemicals on the market that claim to help clarify aquarium water which could possibly help eliminate cloudy water (generally these products help the free floating particles cling together and get trapped in the filter media). Adding in additional active carbon into your filter may help remove discoloring and smell from the water, but it can also remove chemicals. Certain medications (such as Methylene Blue) can cause cloudy water because it killed off any beneficial bacteria the filter system may have had and now the tank is going through bacterial bloom because it needs to go through the water cycling process again.
6. Cycle: Sometimes in new aquarium setups where water hasn't gone through the cycling process it can cause the aquarium water to get cloudy from 'bacterial bloom'. Bacterial Bloom is generally cloudy white in color, compared to Algae Bloom that is generally cloudy green. Once the filter media gains enough beneficial bacteria it can help combat some possible cloudy water issues. Be sure to check your aquarium for nitrite and ammonia during this time as these can be deadly to your aquariums' inhabitants. To help speed up the process there are various water chemicals you can add, although I personally prefer using live bacteria balls in my canister filter. I have also had success using Tetra SafeStart which I find to be readily available in most stores with aquarium goods.
7. Oxygenate: If your aquarium water continues to be cloudy or has a very dense amount of cloudiness, then you may want to add in an air diffuser to ensure the fish are at least getting adequate amounts of oxygen in the water while you continue to remedy the cloudy water. Algae bloom or 'green water' can cause fish to suffocate and die without added oxygenation. You'll want an air diffuser that gives off fine air bubbles, as larger bubbles usually just agitate water at the surface. I suggest using a wooden air diffuser such as those made by Lee's. Also, if you can attach the air diffuser to the center (or a bit off center) base of the tank rather than the sides, it allows the air bubbles to fully disperse within the tank. If you have substrate than this will be harder for you to accomplish and instead just attach the diffuser to the widest wall of the tank while trying to center it close to the base of the aquarium. Doing this will help fan out the air bubbles while giving the bubbles more time burst before hitting the surface.
8. UV Filter: When all else fails in terms of combating a cloudy tank try using a UV filter system. A UV filter helps to sterilize tank water and is often used to remove algae bloom, although don't expect this to remove algae attached to surfaces in the aquarium. Generally, you don't run a UV filter all the time, just when it becomes necessary. A UV filter also won't kill your beneficial bacteria otherwise, you wouldn't be able to use these at all in a tank if that were the case. These units can also be pricey, along with bulb replacement maintenance that you'll probably be doing on them in the future. Another reason not to run a UV filter all the time is because depending on its housing unit it can melt the unit over time and thus leach harmful chemicals into your aquarium.