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? It can be called mulm or detritus or debris and is an integral part of healthy aquariums. Read on to find out what mulm looks like, how it can be removed, and how you can minimize it.

What is Mulm?

Mulm is made from fish poop and plant leaves. The decaying organics are broken down by bacteria, fungi, microorganisms, and tiny microfauna. This army detritivores converts organic matter to mulm. Mulm is rich in nitrogen compounds and essential minerals, which can be eaten by plants and other animals. In fact, the fertile soil in our yards and gardens is basically mulm that is made up of decaying leaves, animal droppings, and so forth. Mulm is a kind of compost heap in an aquarium. It is where organic waste is transformed into rich nutrients that can be used for the revitalization of the substrate in which plants grow.

Is Mulm Good for You?

You can generally say no, as long as there is enough biological filtration (e.g. beneficial bacteria and microorganisms), to safely remove the waste. An aquarium water test kit can help you determine the level of ammonia, nitrite and nitrate in your aquarium. If you have not cycled your tank, then detritus buildup could indicate that your tank has dangerous levels of these toxic nitrogen waste compounds. This can be fatal for your fish. Mulm can look like black or brown sediment. If you notice large amounts of uneaten food, or any other organic matter that isn’t being broken down, it may be time to remove them with a gravel vacuum. This will prevent dangerous spikes in nitrogen waste.

Mulm is a good choice for planted aquariums as it revitalizes the substrate and provides nutrients for plants to eat.

While mulm may look a bit unsightly, it’s actually an indication that you have a thriving ecosystem in your fish tank that can support life and process organic waste without a drop in water quality. For example, ponds and lakes in nature may appear to be “dirty” because of their murky, muddy waters. However, the mulm beneath those waterways is filled with nutrients that continue feeding the living organisms. 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. The tank will look cleaner and clearer if the excess mulm is removed. – Live plants in fish tanks: Mulm is often left in aquariums because it gives plants essential nutrients. Mulm can cover carpeting and foreground plants. If this happens, you might want to remove some of the mulm to ensure your plants receive enough light. – Fish tanks with fry: Mulm in an established aquarium often grows infusoria and other microorganisms that are an ideal first food for baby fish. 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 sucked up.

How do you remove or hide Mulm?

If you wish to remove mulm, it can be easily vacuumed up using an aquarium siphon. Low flow areas are where detritus can build up and accumulate. It also sticks to aquarium decorations, driftwood, rocks, and other small objects. Be careful when vacuuming gravel if you have shrimp or baby fish in your tank. To gently remove any debris, some breeders prefer to use a turkey baster (also known as the siphon tube) or airline tubing.

This method is ideal for aquariums with fish who 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 that are small and tightly-packed (like sand), tend to build up mulm faster because debris cannot get in or be embedded into the sand. Choose a tan-colored substrate that is mottled to blend in with the surroundings. Another solution is to pick a substrate with small, pebble-sized particles (like gravel or Seachem Eco-Complete) that has plenty of gaps in between, thus allowing the mulm to easily sink between them and reach the roots of your plants.

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

You can find more tips and tricks to keep your aquarium looking beautiful and clean in our other maintenance articles: