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. We’ll be breaking down mulm and discussing how to minimize it.

What is Mulm?

Mulm begins as fish poop or plant leaves. The decaying organics are broken down by bacteria, fungi, microorganisms, and tiny microfauna. The army of detritivores transforms organic matter into mulm. Mulm contains essential minerals and nitrogen compounds that can be used by plants and algae. 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 Harmful or Beneficial?

You can generally say no, as long as there is enough biological filtration (e.g. beneficial bacteria and microorganisms), to safely remove the waste. This can be checked with an aquarium water tester kit. It will determine if you have less than 40 parts per million of nitrate, ammonia and 0 parts per million of ammonia. If your tank is not cycled, detritus buildup could be a sign that your aquarium is reaching harmful levels of these nitrogen waste compounds, which can be lethal to your fish. 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 good for planted aquariums, as they rejuvenate the substrate and provide nutrients for plants.

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. Because of their murky and muddy water, lakes and ponds in nature can 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. Here are some different setups to consider:

Fish tanks with no live plants: Mulm can cause the water to become cloudy, particularly if the fish are bottom-dwelling and like to scavenge on 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. If the mulm is too thick to cover your carpeting, or foreground plants, it may be necessary to remove it. This will ensure that the plants get 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 can you hide or remove mulm?

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

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. The aquarium filter will collect the debris and then force it into the water column. Filter clogs can be caused by too much mulm. If it is a hang-on back filter, the filter may overflow.

If you have a planted aquarium and want to keep mulm in the substrate, there are ways of minimizing its appearance so that your fish tank doesn’t 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. You should choose a dark, mottled substrate to camouflage the mulm and blend into its surroundings. Another option is to select a substrate with tiny, pebble-sized particles like gravel or Seachem Eco-Complete that has lots of gaps between. This allows the mulms to easily sink between them, and reach your plants’ roots.

Gravel-like substrate with a varied brown color is great at camouflaging and incorporating mulm particles.

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