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Hard Water
Frequently changing the water in an aquarium doesn't guarantee the glass won't turn cloudy, because sometimes the water itself causes the problem. Carbonates or calcium in hard water can bond with silica in glass, forming a thin film of limestone. You can use a 1:3 solution of vinegar and warm water to scrape off the limestone film. After applying, ensure that your fish tank has the required conditions to avoid limestone buildup again, such as optimum water pH, nitrite and nitrate levels.
Defective Glass
Defective glass can also cause cloudy formations on top of an aquarium. Cited as crackle defect or "crazzling," this is due to faulty glass manufacturing practices. It may be irresolvable in some cases. Replacing the tank may be the only solution, aside from having the glass melted. Both can cost money.
Detergent
Cleaning the glass with abrasives or leaving traces of detergent may also cause cloudiness in an aquarium. Soap residue may form a film on the glass tank due to oxidation. This can be the least difficult cause; it only requires polishing. Aquarium cleansers can easily remove this kind of stain. However, using soap or other types of abrasives also endangers the fishes' health.
Fish Food
Putting too much food into the tank may also cause cloudiness because excess food gathers on top of the water and clings onto the glass. As water evaporates, this excess food dries and hardens. If the cloudiness is caused by excess food, it can easily be scraped off with an abrasive material. To avoid this problem, regulate feeding time and make sure you give fish food that can only be consumed within five to 10 minutes.