Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Green water is almost always because of a type of algae that drifts in the water. It can look cloudy, but not always green in the tank. As the algae grows it gradually looks greener. It can get as dense as pea soup.
Other reasons for cloudy water might also be involved, but they would not look green.
Ammonia rising, or something rotting in the tank. Any dead fish? Excess food? There may also be bubbles at the surface that tend not to pop very easily. There might be a surface scum at the top. Not always.
pH adjusting chemicals can make the water cloudy. pH Up or pH Down and similar are rather tricky to use without understanding the chemistry behind them.
Mineral additives like certain GH boosters can look cloudy until they dissolve. Usually this is temporary.
Heterotrophic bacteria can grow so fast they cloud the water. These are not the nitrifying bacteria, but they are not harmful. They usually go away when their food source is gone. This sort of cloudy water is common in a new set up, or in an old tank if you have disturbed something like the substrate and added organic matter to the water. These bacteria are not green, but green water algae can also grow when nutrients are added to the water, so I would not be surprised if both were present at the same time.
Substrate is getting stirred up. Any digging fish?
Odd things can also create cloudy water, and I have heard about some really odd things like Jell-O, vitamin tablets, toys and a lot more.