Why Does My Betta Tank Have An Oily Film On Surface? (Solved!)

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Have you looked into your Betta’s fish tank and noticed a thin, oily film on the surface?  Does it matter? Is it harmful to your Betta? In this article, I look at what causes an oily film on the surface of a Betta tank and how we can prevent it from happening.

The thin, oily film that often forms on the surface of a Betta tank is caused primarily by proteins in the food we feed to our Bettas.  Lack of surface agitation allows the proteins to build up to form the oily film. The film then prevents the proper exchange of oxygen between the water and the air, leading to low oxygen levels in the aquarium water.

Why does my Betta tank get an oily film on top?

The main causes of oily films on the surface of a Betta tank are;

  • Fish Foods
  • A dead and decaying fish
  • Oil from fish keepers hands
  • Fish waste
  • New equipment
  • External environmental factors

Whilst there can technically be a number of different causes of oily films on the surface of a Betta tank, the most likely cause is proteins contained within the foods we feed our fish.

Every food we feed to our Betta’s, whether it be a store-bought pellet, a live or frozen food like bloodworms, or a homemade concoction, contains proteins.   These proteins are released naturally both whilst the Betta eats them and when they go to the bathroom.

Fish Foods

Every food we feed to our Betta will essentially release proteins into the water when a Betta eats it, although some are worse than others. 

Frozen bloodworms are well known to be high in protein, and so often a culprit of causing the protein film.  There are also some flake foods on the market which have a distinct ‘oily’ feel about them.

There isn’t a great deal you can do to prevent proteins from being released from foods, although avoiding the cheapest foods on the market will no doubt help.  I like to feed my Betta’s Vibra Bites from Hikari and Bug Bites which is made by Fluval (I ordered my Bug Bites from Amazon – check their current price).  Both foods have high-quality ingredients and are relished by almost all my Betta’s.

Dead or Decaying Fish

When a fish dies in our aquariums, their bodies quickly start to break down, especially if there are other tank mates in the aquarium with them that might start to eat the dead fish.  Oils from dead fish are quickly released as the body decays.  

The simple way to prevent a dead fish from creating an oily surface is to remove the fish as soon as you realize it is deceased. 

Oil From Fishkeepers Hands

As fish keepers, we don’t give a second thought to putting our hands into our Betta’s tanks. We often adjust a heater, move a rock or maybe stand up a decoration that has fallen over.  The problem is, we then transfer whatever is on our hands straight into the aquarium water.  This may include lotion from washing our hands.

Unless you have to put your hands into the tanks in an emergency situation, try to get into the habit of washing them first, but just in running water, without using any detergent or lotion.    

Fish Poop!

Needless to say, when a fish goes to the bathroom their waste will inevitably release some oils back into the aquarium and these oils will float to the surface to create a film.  Unfortunately, there isn’t much you can do about this one!

New Equipment

It often surprises people when I suggest a new filter, heater or even the inside of a brand new aquarium may have an oily film as a result of manufacture and transportation.  

Whenever I buy a new piece of equipment I wipe it down with paper towels to remove any contaminants that may affect my aquarium water.  Before I fill a new aquarium with water I always wash it out and rinse it really well.  Next time you buy a new aquarium, just run a clean paper towel around the inside.  You will be amazed how dirty it comes out.

Environmental Factors

I went through a phase of keeping tanks without lids on.  The water surface was completely open.  The effect is mesmerizing.  However, tanks without lids are magnets for everything floating around it the room, including dust, pollen, and spray from deodorants or room sprays.  whatever is in the atmosphere around your tank will eventually end up on the water’s surface.

Preventing a surface film on a lidless tank is easy, put a lid on it.

Is the oily film bad for my Betta?

Having an oily film on the surface of a Betta tank doesn’t look very nice, but is it actually bad for a Betta?

Having an oily film doesn’t directly cause a Betta any real issues.  It won’t stop a Betta from eating and a Betta can easily just push its nose through the film if it needed to. 

The oily film can however stop the natural exchange of gases between the aquarium water and the air above.  This gas exchange is essential for allowing oxygen to dissolve into the aquarium water and carbon dioxide to leave the water.

Although Betta’s do have the ability to breathe directly from the air if they have to, they really would rather just absorb oxygen from the water through their gills.  A very thick oily film can lead to the oxygen levels in the aquarium becoming low, causing the Betta to be in a stressful situation.

How do you get rid of the oily film in an aquarium?

The simplest way to remove the oily film on the surface of a fish tank is to carefully place a paper towel on the surface of the water.  After a couple of seconds, slowly remove the paper towel.  Repeat until all the oily film has been removed.

When using this technique, do bear in mind not to stir the surface, otherwise, the oily film will get mixed back into the aquarium water. Also, if you do have mechanical surface movement, such as a filter or powerhead, remember to turn them off and allow the water to settle before using paper towels.

Using a paper towel to remove the oily film is a great short-term solution, but in the long run, the film will simply return.  

How do you prevent an oily film from forming?

When it comes to preventing an oily film from forming, surface movement is the answer.  The more surface movement you can generate, the less likely it is an oily film will form.

There are a number of different ways to create sufficient surface movement that an oily film won’t form in the first place.  These include;

  • An airstone
  • A powerhead
  • A surface skimmer
  • Adjusting the filter return

An Airstone

Placing an airstone, which is connected to an aquarium air pump, into a Betta tank is by far the simplest and cheapest way to prevent an oily film from forming. As the bubbles pour out of the airstone, they race to the surface where they pop! It is this popping action that causes the surface film to break up allowing it to be drawn into the aquarium filter.

I run an airstone in every Betta tank I own.  Actually, I run an airstone in EVERY tank I own, no matter what the inhabitants! I am a big fan of these airstones on Amazon.

A Powerhead

An aquarium powerheads job is to draw water in through its inlet, then pump it out through a return spout. Sometimes powerheads are attached to sponge filters or undergravel plates, and sometimes they are purely used for water circulation.

By placing a powerhead so the return spout is located at the right point that the returning water agitates the surface as it rushes out, you break up any oily proteins before they have a chance to form into an oily film.

A Surface Skimmer

An aquarium surface skimmer is a piece of equipment designed for one job, to skim the water surface, removing proteins before they form into an oily film.  Surface skimmers can be dedicated pieces of equipment, attachments to existing filters, or homemade devices.

Personally, I have had great success using surface skimmers that attach to a canister filter.  They work by drawing in a small amount of water from the surface whilst the canister filter still takes in the majority of the water from the usual filter intake.

I am not much of a DIY man, but the video below shows how to make a DIY surface skimmer.

Adjusting A Filter Output

If you are already using a canister filter, hang-on-back filter, or internal power filter, you can usually adjust the filter output to increase surface agitation.  Whatever method the filter uses to return the water to the aquarium, simply move it up or down as necessary to create maximum surface movement.

My final thoughts on ‘Why does my Betta tank have an oily film on the surface?’

Oily films can easily form on the surface of a Betta’s tank. The reason they form is almost always down to lack of surface movement.  My first recommendation would ALWAYS be to add an airstone.  Alternatively, adding a surface skimmer or adjusting the filter return can also solve the problem.

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James