Why Are There Foamy Bubbles in My Fish Tank?

Causes of Tiny, White Bubbles on Glass and the Surface

Foam on Aquarium Water

Illustration: The Spruce / Emily Mendoza

Are there bubbles or foam floating on top of your aquarium water? A bubbly surface may happen overnight and is a common concern for aquarists. Fish tank foam can come from many sources; some are harmless while others can be deadly. Here are the causes of bubbles in aquarium water and solutions to eliminate them.

What Are Foam Bubbles?

Although they can be caused by different factors, foamy bubbles are all tiny capsules of gas (oxygen or carbon dioxide) trapped in a film made of one of the following substances:

  • Agitated liquid (water)
  • Soap
  • Protein (dissolved fish poop, food, and other organic waste products)
  • Fish saliva

Agitated Water

Bubbles often form when liquids are agitated; they are simply oxygen-filled water pockets. In an aquarium, this can happen in a couple of different scenarios:

  • When filling or topping off an aquarium, it's not unusual to agitate the water enough to cause at least some foam, which is normal and not harmful. To avoid bubbles when you add water to a tank, pour the new water slowly against a clean plate held at an angle so that the water runs down the plate and into the tank rather than splashing forcefully into the aquarium.
  • If you have a spray bar or powerheads, these may agitate the water sufficiently to produce some foamy bubbles. Generally, this pressure-generated foam will not occur in large quantities and is composed of large bubbles that dissipate quickly.


Sometimes, soap gets into an aquarium via cleaning materials, such as the water bucket or scrubbing pads. If even a small amount of soap or cleanser finds its way into the aquarium, it will usually cause sudsy rainbow-tinted bubbles to form (like the bubbles on a sink full of soapy water). This type of foam is dangerous because it can kill fish.

  • To save your fish, immediately move them to another tank or a clean bucket filled with dechlorinated water at the same temperature as the aquarium. Drain the soapy aquarium and rinse all components thoroughly. Strip the filter as well, replacing all filter media and pads which will have collected soap residue. Refill the aquarium and move your fish back into the tank.
  • To avoid introducing soap into your aquarium, designate a set of tank cleaning tools that are not used for anything other than aquarium cleaning.


Protein foam is the result of organic waste (feces, dead fish, and uneaten food) forming an oxygen-trapping surface film that creates small air bubbles that stick together and form a smelly foam. It is more prevalent in saltwater aquariums but is seen occasionally in freshwater tanks as well. Protein foam is not an emergency, but it needs to be addressed because it can decrease your tank's oxygen over time.

  • If protein foam is present, it is an indication that the aquarium needs a good cleaning. Make sure the filter is clean, and remove any debris in the aquarium using a gravel vacuum.
  • Check for any dead fish that may have settled in a hidden corner, behind plants or rocks. A disintegrating fish corpse is a rich source of protein, and frequently the root cause of protein foam. Decaying plants can also produce protein wastes, so trim dead leaves off your aquatic plants and remove them from the aquarium.
  • Saltwater aquarium systems can incorporate protein skimmers; this process removes excess proteins from the surface of the water. However, protein skimmers are not effective in freshwater aquariums.
  • Regular water changes, filter maintenance, and routine gravel cleaning will ensure that you do not have a problem with protein foam.


If you recently acquired a male Labyrinth fish, such as a Betta or Gourami, patches of new foam might be a bubblenest produced by the male fish. These species blow bubbles of sticky saliva into a floating mat or nest of foam to attract females (even if there are no females in the tank).

A bubblenest indicates that your male fish is content and healthy, and it is not a health threat to other fish. Try not to disturb bubblenests. Even if there is only a single fish in the tank, a disrupted nest can cause stress to its dedicated builder.

Fish Medications

Medications such as antibacterial treatments can change the water chemistry of an aquarium, encouraging a foamy film of organic waste products to accumulate on the surface of the water. This foam is not harmful and should dissipate within a few days of ending the course of treatment.


Tiny, clear bubbles that form on the leaves and stems of aquatic plants are carbon dioxide-filled water capsules that occur during photosynthesis. Seasoned aquarists call this "pearling" and consider it to be a sign of excellent plant health. These bubbles are not harmful to fish.