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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? This dirt-like material goes by many names – such as mulm, detritus, and debris – and it’s a naturally occurring 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 starts off as fish poop, plant leaves, leftover fish food, and other organic materials that are decomposing in the water. The decaying organics are broken down by bacteria, fungi, microorganisms, and tiny microfauna. This army of detritivores turns the organic matter into mulm, which contains nitrogen compounds and essential minerals that can be consumed 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. 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?

Generally speaking, no – as long as you have enough biological filtration (e.g., beneficial bacteria and microorganisms) to safely break down the waste. You can measure this with an aquarium water test kit to make sure you have 0 ppm (parts per million) ammonia, 0 ppm nitrite, and less than 40 ppm nitrate. 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.

planting hair grass into planted aquarium substrate

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

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 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.

Should You Get Rid of Mulm?

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. 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. Plus, the extra debris provides extra cover for smaller fry.
aquarium siphon

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?

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 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 next method is great for aquariums with fish that can swim in high currents. Increase the water flow in the fish tank using power heads or circulation pumps. 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. 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 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. Therefore, choose a mottled, tan-colored substrate so that the mulm is camouflaged and blends in with its 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.

planted tank with gravel

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

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

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