What is Mulm or Detritus in Aquariums?

Is there a brown or black substance that seems to collect like dust bunnies all over the floor of your fish tank? This dirt-like substance is known by many names, including mulm, debris and detritus. It is a natural part of healthy aquariums. Continue reading to learn more about mulm, its composition, how to reduce it and whether you should get rid of it.

What is Mulm?

Mulm begins as fish poop or plant leaves. The bacteria, fungi and microorganisms break down the organic matter. This army of microorganisms transforms the organic matter into mulch, which is rich in essential minerals and nitrogen compounds. The fertile soil that we have in our gardens and yards is actually mulm, which is made up of animal droppings and decaying leaves. Therefore, think of mulm is like the compost heap of an aquarium, where organic waste turns into compost that is rich in nutrients and can be used to revitalize the substrate that plants grow in.

Is Mulm Harmful or Beneficial?

The answer is generally no, provided you have sufficient biological filtration (e.g. good bacteria and microorganisms), in order to safely eliminate the waste. An aquarium water test kit can help you determine the level of ammonia, nitrite and nitrate in your aquarium. If you do not cycle your tank, then detritus buildup could mean that your aquarium has dangerous levels of nitrogen waste compounds. These can be fatal to fish. Also, remember that mulm looks like brown or black sediment, so if you see large amounts of uneaten food or other organics that aren’t breaking down, consider removing them with a gravel vacuum to prevent deadly spikes in nitrogen waste.

Mulm is beneficial to planted aquariums because they revitalize the substrate and add nutrients for plants to consume.

Although mulm might seem unattractive, it is actually a sign of a healthy ecosystem in your fish tanks that can sustain life and process organic matter without affecting the water quality. Because of their murky and muddy water, lakes and ponds in nature can appear “dirty”. However, the mulm at the bottom of those waterways is packed full of nutrients that continually feed the inhabiting plants and animals in the cycle of life. In fact, some aquarium hobbyists encourage the growth of mulm by adding catappa leaves and driftwood to create a more natural-looking biotope or breed fish that like the additional cover.

Are You Ready to Get Rid of Mulms?

It depends on whether or not your aquarium can benefit from it. Here are some different setups to consider:

– Fish tanks without live plants: Mulm can make the water a little cloudy, especially if you have bottom-dwelling fish that like to scavenge in the substrate. Mulm can cause the water to cloudy and make the tank appear cleaner. – Fish tanks that have live plants: Detritus can be left in an aquarium to provide essential nutrients and reduce the need for fertilizer. However, if there is so much mulm that it covers your carpeting or short foreground plants, you may want to remove some of it to make sure the plants are getting enough light. Fish tanks with fry Mulm is a good choice for babies fish as it can grow infusoria or other microorganisms in aquariums. The extra debris also provides additional cover for the smaller fry.

An aquarium siphon is a device that vacuums the bottom of fish tanks. The heavier substrate sinks to it while the lighter mulm is absorbed.

How do you remove or hide Mulm?

If you wish to remove mulm, it can be easily vacuumed up using an aquarium siphon. In low flow areas, debris tends to accumulate at the bottom. It also gets stuck behind aquarium decorations, driftwood, and rocks. If you have baby fish or shrimp in the tank, be very careful when gravel vacuuming. Some breeders prefer to use a turkey baster or airline tubing (as the siphon tube) to gently remove debris.

This is a great option for aquariums that have fish that can swim in high currents. Power heads and circulation pumps can increase the flow of water in the fish tank. By blowing the detritus into the water column, it has a greater chance of being sucked up by the aquarium filter so that the particles can be mechanically strained out of the water before returning to the fish tank. It is possible for the filter to become clogged if there is too much mulm.

There are many ways to minimize the appearance of mulm in a substrate for a planted aquarium. This will ensure that your fish tank does not look dirty. Substrates with small, close-fitting particles (like sand) often build up mulm more quickly because the detritus cannot enter or get embedded into the sand as easily. Choose a tan-colored substrate that is mottled to blend in with the surroundings. A second option is to choose a substrate that is small and pebble-sized (such as gravel or Seachem Eco-Complete). This will allow the mulm to sink between the particles and reach the roots of the plants.

Gravel-like substrate in a variegated brown color is ideal for concealment and incorporation of mulm particles.

For more tips and tricks on keeping your aquarium looking clean and beautiful, check out our other tank maintenance articles here: