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.

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. 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

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 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”. 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. 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. The water will look clearer and cleaner if you remove excess mulm. – Fish tanks that have live plants: Detritus can be left in an aquarium to provide essential nutrients and reduce the need for fertilizer. You may need to remove mulm from your aquarium if it is covering your carpeting or other foreground plants. – 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. Additionally, smaller fry will benefit from the extra debris.

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. Low flow areas are where detritus can build up and accumulate. 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. 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. You can increase the water flow to your fish tank by using power heads or circulation pump. The aquarium filter will collect the debris and then force it into the water column. It is possible for the filter to become clogged if there is too much mulm.

If you have a plant aquarium, and wish to keep the mulm in its substrate, there are several ways to reduce its appearance. 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. Choose a tan-colored substrate that is mottled to blend in with the 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.

Gravel-like substrate in a variegated brown color is ideal for concealment and incorporation of mulm particles.

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