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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? Mulm, debris, and other dirt-like substances are all names for this substance. It’s an important part of healthy aquariums. We’ll be breaking down mulm and discussing how to minimize it.

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What is Mulm?

Mulm starts off as fish poop, plant leaves, leftover fish food, and other organic materials that are decomposing in the water. 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. 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?

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. 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. 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.

Mulm can be a little unsightly but it is a sign that your fish tank has a healthy ecosystem that can sustain life and eliminate organic waste. 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.

Should You Get Rid of Mulm?

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

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. Removing the excess mulm will help keep the water clearer and the tank look 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 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 can be used to vacuum the bottom of a fish tank because the heavier substrate sinks to the bottom while the lighter mulm gets sucked up.

How do you remove or hide Mulm?

You can remove mulm with an aquarium siphon. In low flow areas, debris tends to accumulate at the bottom. It can also stick to aquarium decorations, driftwood, or rocks. If you have baby fish or shrimp in the tank, be very careful when gravel vacuuming. To gently remove any debris, some breeders prefer to use a turkey baster (also known as the siphon tube) or airline tubing.

This is a great option for aquariums that have fish that can swim in high currents. You can increase the water flow to your fish tank by using power heads or circulation pump. To ensure that debris is not clogged in the water column, blow it around. The aquarium filter can then collect the debris from the water and strain it out. 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.

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 (such as sand), tend to build up mulm faster because debris cannot get into or embed itself in the sand. 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.