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. Keep reading as we dissect what mulm is made of, whether you should remove it, and how to minimize its appearance.

What is Mulm?

Mulm is made from fish poop and plant leaves. The microorganisms and bacteria that break down decaying organics can be used to make mulm. This army of microorganisms transforms the organic matter into mulch, which is rich in essential minerals and nitrogen compounds. Mulm is essentially decaying leaves and animal droppings that makes up the soil in our gardens and yards. 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?

Generally speaking, no – as long as you have enough biological filtration (e.g., beneficial bacteria and microorganisms) to safely break down the waste. An aquarium water test kit can help you determine the level of ammonia, nitrite and nitrate in your aquarium. Detritus buildup in your tank could indicate that you have too many nitrogen waste compounds. This can cause fish to become sick. Mulm can look like black or brown sediment. If you notice large amounts of uneaten food, or any other organic materials that aren’t being broken down, it is worth removing with a gravel vacuum. This will prevent dangerous 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. A murky, muddy environment can make ponds or lakes appear dirty. But the mulm at their bottom is rich in nutrients that sustain the life cycle of the aquatic plants and animals. 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.

Do You Need to Get Rid Of Mulm?

It all depends on if your aquarium can use it. These are just a few of the options:

– Fish tank without live plants: Mulm may cause water to cloudy, especially for bottom-dwelling fish who like to dig in the substrate. Mulm can cause the water to cloudy and make the tank appear cleaner. – Fish tanks with live plants: Detritus is often left in the aquarium because it provides essential nutrients for plants to feed on and can potentially decrease the amount of fertilizer that is needed. 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 that have fry: Mulm can be found in established aquariums and is often a source of infusoria, microorganisms that make a great 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 absorbed.

How do you remove or hide Mulm?

You can remove mulm with an aquarium siphon. Low flow areas are where detritus can build up and accumulate. It can also stick to aquarium decorations, driftwood, or rocks. Be careful when vacuuming gravel if you have shrimp or baby fish in your tank. To remove small pieces of debris, some breeders prefer to use a turkey baster or airline tubing (as a siphon tube).

This is a great option for aquariums that have fish that can swim in high currents. Increase the water flow in the fish tank using power heads or circulation pumps. The aquarium filter will collect the debris and then force it into the water column. If too much mulm builds up in the filter, it may become clogged (and even overflow if it’s a hang-on-back filter), so make sure to regularly clean your filter and rinse out the accumulated sludge.

If you have a plant aquarium, and wish to keep the mulm in its substrate, there are several ways to reduce its appearance. 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. Therefore, choose a mottled, tan-colored substrate so that the mulm is camouflaged and blends in with its 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 substrata with a brownish color is great for hiding mulm particles and camouflaging.

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