White film often builds up in aquariums and may collect along the glass or on plants. The cause can range from completely harmless bacteria to potentially deadly ich. In order to treat the film, carefully examine the aquarium environment and look at potential sources of disease.
If your aquarium water is cloudy and you are noticing white film on the sides of the tank, your water is dirty. Start by completely changing the water and cleaning the filters. You may want to consider increasing filtration by using two aquarium filters. Inadequate filtration can contribute to dirty water. Only feed your fish as much as they will eat in 15 minutes. Dirty water is often caused by overfeeding or infrequent water changes.
If your aquarium water is clear and your fish are healthy, the white film may be bacteria growing. Aquariums tend to cycle bacteria and it's not unusual for a newly-established tank to suddenly begin growing bacteria. Allow the tank a few days to cycle through the white film. If the film does not go away on its own, change the water and clean the filters. Avoid changing the gravel, which can cause the tank to begin a new cycle of bacteria growth.
Aquariums are mini ecosystems that process and filter a variety of nutrients and minerals. Occasionally these minerals build up along the sides of tanks and even over light fixtures and in cracks. If you've used a lot of chemical treatments in your aquarium lately, this could be the cause. Check for a dry, flaky white crust that can only be removed by scraping. If the buildup is caused by minerals, the film itself is harmless. It may, however, serve as an indication that you are over-treating your aquarium. You can get rid of the film by using an aquarium scraper or by changing the water and rinsing your aquarium with a mild detergent soap. Get rid of all of the soap before putting your fish back in your tank.
If the buildup is fuzzy and full of filaments and it tends to float mostly in the water, only occasionally building up on the glass, it may be mold or fungus. Start by changing the water and carefully cleaning the filters. Then treat your entire aquarium using mold and fungus pellets from your local pet store. It's necessary to treat your tank even after cleaning the water because any fungus left behind will quickly grow again and can endanger your fish.
If there are small white spots on your aquarium glass as well as on your fish, your fish almost certainly have ich. Ich is often deadly and must be treated immediately. Ich often grows in water that is too cold. Check the requirements for your fish and raise the water temperature if necessary. You should also treat your fish for ich immediately. You can buy treatment tablets at most pet stores. Carefully follow the directions and change the water twice a week until the infection is gone.