We have had many readers reach out to us reporting that there is a blue film forming on top of their fish tank water. Most of the time, this looks like gasoline on top of a road after it rains. Aside from blue, this oily residue can also be green, yellow, or even the color of the radio. There are many reasons why blue film forms on top of fish tank water, and in today’s article we will explain all those reasons.

Blue film on top of fish tank water

The main reason blue film forms on top of fish tank water is because of a chemical imbalance. This is usually an indicator that your fish tank water is unhealthy. The main causes of a chemical imbalance and blue film is excess organic waste, algae, low water flow, and oil extracted from your hands.

Organic waste

Organic waste is the number one cause of blue film forming on top of your fish tank water. The most common organic waste in a fish tank is excess food and fish poop. There can also be organic waste from dying plants. While a fish tank can usually absorb most of this waste, an excess amount can result in the blue film forming on top of the water.

For this reason, it is important to only feed your fish the serving size listed on the bottle, as the uneaten food in the fish tank will certainly cause a build up of organic waste. We also recommend removing aquarium plants that are dying and starting to decompose.

Algae

Algae is another main cause of blue film in fish tanks. This is most common with the blue-green algae, but it can also occur from other types of algae and brown diatoms.

While small levels of algae are okay for your fish tank, an excessive amount can be very dangerous for your fish. If the algae is sitting at the top of the water, it can block sunlight from the water, which can significantly decrease the oxygen level of your fish tank.

In turn, your fish will have a very hard time breathing. For this reason, we recommend always removing algae if it is floating at the top of your fish tank. One way to slow down the growth of algae in your tank is to decrease the brightness of your fish tank light.

Low Water Flow

Low water flow or aeration is another cause of blue foam in your fish tank. If the water in your fish tank is not circulating enough (especially at the top of the water), there is a good chance blue film will form at the top. Circulation is important not only to keep blue film oil from forming, but it also helps increase the oxygen level in your tank.

If you notice that your water is sitting still, you should increase the flow rate in your filter. If you do not have a filter, we recommend buying an aerator to increase water flow and oxygen. You can also look into buying a fish tank skimmer which will help regulate the nitrate levels in your tank.

Oil from hands

An often-overlooked danger to your fish tank water is your own hands. As you probably know, many forms of soap are extremely toxic to fish. Even if you don’t notice it, the oil from certain soaps can stay in your pores even hours after your wash your hands. The same goes for lotion and moisturizers.

When your hand enters the fish tank, the water automatically opens up your pores, and the soap and oils can then enter the water in your fish tank. This oil can form together, which will also cause the blue film on top of your fish tank. For this reason, we recommend always washing your hands with warm water before you stick your hands in your fish tank.

What to do if there is blue film on top of your fish tank water?

The best thing to do when you find blue film on top of your fish tank water is to remove it. We suggest doing this with a paper towel, as it is able to soak up the oil from the water. It is important to remove the oil quickly, as a big enough oil build up can make it very hard for your fish to breathe. After removing the blue film, you should consult the preventive measures listed above.