What Causes Scum in an Aquarium?

Although the scum on your aquarium walls resembles the grime on your shower floor, aquarium scum is more complex than it appears. Algae, also known as "scum," is a living organism that travels through tap water and thrives under the right conditions inside a fish tank. The scum that accumulates on your tank walls is the result of algae blooms and waste produced by fish and plants inside the aquarium. Identifying the causes of aquarium scum will help prevent its buildup.

  1. Fish Food

    • Fish food is nutrient-dense and water-soluble. This makes it the perfect nutrient source for algae and aquarium scum. Fish food ends up in one of two places: in the stomachs of your fish or the bottom of the tank. The uneaten food flakes settle to the bottom, where they become a pulpy, gelatinous resin that clouds the water and fuels algae growth. Ideally, your fish should consume all food flakes within 30 seconds after the food is dropped into the tank. Food flakes falling to the bottom of the tank may indicate that you̵7;re overfeeding your fish.

    Nitrogenous Waste

    • Nitrogenous waste is a chief cause of algae and aquarium scum. Warm water mixed with waste from plants and fish creates an optimal environment for nitrogenous waste. Over time, the high nitrate levels in your aquarium fuel algae growth. Regularly changing your tank water and installing a high-powered filtration system will reduce the scum and algae by regulating the nitrate level in the water.

    Aquarium Lighting

    • Aquarium lighting requires a precarious balance. Like any plant, algae needs light to thrive, and leaving the aquarium lights on for 20 out of 24 hours fosters algae growth. This blooming algae makes an effective sitting surface for other types of unsightly scum, such as fish excrement. Though light isn̵7;t the sole cause of algae, it helps increase the growth rate. To prevent excess scum and algae growth, limit the use of your aquarium lights to 10 hours or fewer per 24-hour period.

    Phosphates

    • Phosphates exist in tap water, low-grade fish foods and chemicals designed to balance the pH of your aquarium water. Phosphates also feed a specific type of bacterial algae known as cyanobacteria. This bacterial algae resembles red, brown or green slime on the walls of your aquarium and in sufficient amounts can poison your fish. Check the ingredients of your fish food for phosphates and consider purchasing a chemical testing kit to regularly monitor the amount of phosphates in your aquarium.