Mulm and Detritus in Aquariums

Fact Checked by
Sheldon Myers, MS / Aquarist

Do you see a dark brown or black substance forming dust bunnies on the floor of your fish tank? This substance is also known as mulm, debris, and other dirt-like substances. It is a necessary component of healthy aquariums. Detritivores are the organisms responsible for breaking down organic matter into Mulm. They include bacteria, fungi, and microorganisms that decompose organic waste and convert it into nutrient-rich compost. Continue reading to learn more about Mulm, its composition, how to reduce it, and whether it is necessary to get rid of it.

What exactly is Mulm?

Mulch is the decaying leaves and animal droppings that make up the soil in our gardens and yards. Mulm is a product made of fish poop and plant leaves. Bacteria, fungi, and microorganisms decompose organic matter. Organic matter is converted into Mulm by an army of detritivores. Mulm contains minerals and nitrogen compounds that plants and algae can utilize. Mulm is similar to an aquarium compost heap in that it is made up of organic waste that decomposes into nutrient-rich compost that can be used to rejuvenate the substrate in which the plants grow. Mulm can come in different colors and can be formed from different sources such as fish poop and plant leaves.

Is Mulm Beneficial or Harmful?

No, as long as there is enough biological filtration (e.g., beneficial bacteria and microorganisms) to safely remove the waste. An aquarium water tester kit can be used to check this. It will determine if your nitrate, ammonia, and 0 parts per million levels are less than 40 parts per million. Detritus accumulation in your aquarium may indicate that it is not being cycled. This can result in dangerous levels of nitrogen waste compounds, which can kill your fish. Mulm can appear as black or brown sediment. If you notice large amounts of uneaten food or other organics, use a gravel vacuum to remove them. This will prevent dangerous nitrogen waste spikes.

Mulm is an excellent choice for planted aquariums because it revitalizes the substrate while also providing nutrients for the plants to consume.

Mulm can be unsightly, but it indicates that your fish tank has a healthy ecosystem capable of supporting life and eliminating organic waste. Natural lakes and ponds can appear “dirty” due to their murky and muddy water. The Mulm beneath these waterways is rich in nutrients, which continue to nourish the animals and plants. Mulm’s growth and development are encouraged by aquarium hobbyists. To make it look more natural, they add driftwood and catalpa leaves, or they breed fish that prefer the extra cover.

Do You Need Mulm Removed?

It all depends on whether it will benefit your aquarium. These are just a few examples:

  • Fish tanks with no live plants: Mulm can make the water too cloudy, which is especially problematic for bottom-dwelling fish that like to dig in the substrate. If the excess Mulm is removed, the water will appear clearer and cleaner.
  • Live plant aquariums: Mulm is occasionally left in an aquarium to provide essential nutrients and reduce the need for fertilizer. It may be necessary to remove some of the Mulm if it is covering your carpeting or foreground plants. This will ensure that the plants receive adequate light. –
  • Fish tanks containing fry. Mulm can support the growth of infusoria and other microorganisms in a well-established aquarium. This makes it an excellent first food source for young fish. The extra debris also acts as extra cover for the smaller fry.

An aquarium siphon is a device that vacuums the bottom of fish tanks by sinking the heavier substrate and lifting the lighter mulm.

How do you get rid of or hide Mulm?

Mulm can be easily removed using an aquarium siphon. Debris tends to accumulate at the bottom of low-flow areas. It can also become entangled in aquarium decorations such as driftwood and rocks. If you have shrimp or baby fish in your tank, be cautious when vacuuming the gravel. To gently remove debris, some breeders use a turkey baster (also known as a siphon tube) or airline tubing.

This is an excellent choice for aquariums with fish that can swim in strong currents. Water flow in the fish tank can be increased by using powerheads and circulation pumps. The debris will be collected by the aquarium filter and forced into the water column. Mulm buildup can cause filter clogs. The filter may overflow if it is a hang-on back filter.

Mulm can be reduced in a planted aquarium substrate in a variety of ways. This will keep your fish tank from looking dirty. Mulm can be built up more quickly in substrates with smaller, tighter particles, such as sand. This is because detritus cannot quickly enter or become embedded in the sand. To blend in with the surroundings, choose a tan-colored, mottled substrate. A second option is to use a substrate that contains small, pebble-sized particles, such as gravel or Seachem Eco-Complete. This allows the Mulm to sink between the pieces and reach the plants’ roots.

Elliot Galindo
Elliot Galindo
Elliot Galindo is a highly educated expert on freshwater shrimp and their care as pets. He received his Bachelor's degree in marine biology from the University of Oregon and has used that knowledge to become an authority on shrimp care.



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