What Causes Scum in an Aquarium?

Aquarium scum can be caused by a number of factors.
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Maintaining an attractive and healthy environment inside your aquarium for fish can be challenging. Scum on the aquarium walls or on the water’s surface adds to that challenge. Dealing with scum requires identifying its cause from a number of possibilities, including algae and oily foods. Once you've identified the cause, eliminating the scum and preventing it from returning are easier.

Oily Scum

If the scum on top of your aquarium's water seems oily, the problem is probably caused by oily fish or human food. Deep-frying foods in your home can produce vapors that deposit oily residue on the water's surface. Less frying will help prevent the scum from accumulating in the future. Also, frozen varieties of fish food often release oil to the water's surface. In that case, switching fish foods can be an easy solution to the scum problem.

Organic Matter Scum

Detritus, the scientific term for organic matter such as feces, can cause scum to build up on the water’s surface along with dissolved organic matter. While your aquarium’s skimmer removes some of the dissolved organic matter, its design prevents it from effectively removing detritus and all of the dissolved organic matter. You can solve this problem by purchasing a canister filter and using an attachment that will take the surface water back to the filter in order to remove the deposits from the aquarium more efficiently.

Blue-Green Algae

Although lumped together with algae, the blue-green variety is actually cyanobacteria, too much of it in your aquarium can be dangerous for your fish. If the scum on your water is slimy and has a red, black, brown or turquoise coloring, you're probably dealing with a bloom of blue-green algae. Many factors can cause the formation of cyanobacteria in an aquarium, including too much fish food, improper maintenance of the aquarium, poor water circulation and insufficient aeration. You may need to use trial and error to determine what is causing your specific outbreak of blue-green algae so you can correct the problem and prevent its return.

Other Types of Algae

"Algae" is really an umbrella term for many different types of plant life that exist in the water. Most aquariums that provide an adequate living environment for fish will become home to algae at some point. In fact, algae in the aquarium are not necessarily a bad thing, although too many can become a problem for your fish and can minimize the attractiveness of your aquarium. Growing your own macroalgae or seaweed in your saltwater aquarium is one way to reduce algae buildup.

Blue-Legged Hermit Crab

The blue-legged hermit crab can be a solution for many causes of aquarium scum. The crab eats algae and cyanobacteria, so he can prevent both from accumulating in the aquarium. Also, the crab eats the detritus materials left floating in the tank thanks to the fish. By eliminating that organic material from the water, the crab can prevent it from reaching the surface and forming scum. However, the blue-legged hermit crab requires extra shells in the aquarium to keep him from killing any other shelled inhabitants in the tank for theirs.

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