There are a couple of possible causes for hazy water.
Bubbles are noticably visible--you can see them swirling around with any water movement. The bubbles will eventually gas off or attach to the sides/decorations and can be wiped off. Usually, bubbles come from tap water (dissolved gasses that bubble out in normal pressure/temp). Letting water sit overnight will help with this. Powerheads can pull air in, but don't have to run this way.
In a tank that's been running a while, and/or is currently cycling, you will frequently see a small bacteria bloom. This results when free-floating bacteria discover a nutrient rish environment, and thrive. They'll soon consume all the nutrients and starve themselves out--self correcting. Large water changes may decrease the time line for this process, but aren't necessary in a fishlessly cycling tank. If fish are in there, water changes will help prevent oxygen deprivation (the bacteria can consume all the O2, suffocating the fish).
If the haze look green when compared to a white background, it's likely that you have an algae issue. Solutions include adding plants to outcompete the algae for nutrients, several days of 50% water changes, or adding 2-3% water from a healthy planted tank (gaining the benefit of the planktons which normally consume the nutrients feeding the algae bloom).
Are you currently using any additives? Many products marketed to control/manipulate pH contain phosphates, which can cause algae issues.
