What is Mulm or Detritus In Aquariums?

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. 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 begins as fish poop or plant leaves. The microorganisms and bacteria that break down decaying organics can be used to make mulm. 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. 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

The answer is generally no, provided you have sufficient biological filtration (e.g. good bacteria and microorganisms), in order to safely eliminate 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 you do not cycle your tank, then detritus buildup could mean that your aquarium has dangerous levels of nitrogen waste compounds. These can be fatal to fish. Remember that mulm is similar to brown or black sediment. To prevent your fish from dying, remove any uneaten food and other organics with a gravel vacuum.

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

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. Because of their murky and muddy water, lakes and ponds in nature can appear “dirty”. 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. Some aquarium hobbyists encourage mulm growth by adding driftwood and catappa leaf to create a natural-looking biotope. This is also a way to breed fish who like this extra cover.

Do You Need to 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 that have live plants: Detritus can be left in an aquarium to provide essential nutrients and reduce the need for fertilizer. 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.

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. Low flow areas are where detritus can build up and accumulate. It also gets stuck behind aquarium decorations, driftwood, and rocks. You should be careful when vacuuming gravel around baby fish and shrimp. To gently remove any debris, some breeders prefer to use a turkey baster (also known as the siphon tube) or airline tubing.

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. 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. 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. Choose a tan-colored substrate that is mottled to blend in with the 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 substrate in a variegated brown color is ideal for concealment and incorporation of mulm particles.

Check out these other articles for more tips on maintaining your aquarium’s beauty and cleanliness.