The cloudiness you see in your fish tank has a name, infusoria. Infusoria is basically an explosion of microscopic life, which can be unpleasant to look at and could make your fish sick if the growth gets out of control or ends up causing an imbalance in the fish tank environment.
Microscopic organisms will move into the aquarium with any new fish you add -- though it might take two to three days for the bacteria to grow enough to cloud the water. This means you should expect some added cloudiness every time you add something to the tank. Rather than going crazy adding chemicals and moving things around, wait for a few days to see if things settle down on their own. Also, always introduce changes one at a time -- a new fish this week, a new plant next week -- so the environment has a chance to adapt.
Rather than using chemicals to fight cloudiness, change 25 to 50 percent of the water and wait a few days for the cloudiness to clear up. You also can try introducing good bacteria into the tank. Examples of good bacteria include live plants, bacteria-seeded gravel or filter pads, which come covered with healthy microscopic life. You also can move gravel, driftwood or a filter pad from another healthy tank.
Chemicals you add to the water can prolong or worsen the problem. For example, water clarifiers can quickly clear up the water when added to the tank, but they don't solve the underlying issue. Over time, you'll have to add more and more clarifier to keep the water clean while all you're really doing is masking the problem and sometimes even contributing to it.
If cleaning the water and reintroducing health bacteria won't work, the answer might be a much simpler problem. Take a look around the tank and see if you can find an unnoticed dead fish or wasted food floating around. Waste promotes the growth of bacteria, which can contribute to cloudiness. A malfunctioning filter also could be to blame. Check that yours is clean and that water is flowing through it properly. Also look at the manufacturer’s instructions to see if your filter needs any special care or cleaning.