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What is mulm (and how to get rid of it)

If you have a fish tank, you will eventually get mulm.

This guide at Aquarium Blueprints will explain what this substance is in addition to providing the necessary steps to get rid of it.

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What is mulm?

Mulm is a buildup of decomposing organic matter, which includes uneaten fish food, fish excrement and dead plants.

You will most likely find this substance in your filter media (such as sponges, bio balls, ceramic rings and more), on the substrate, decor and/or even plants. It is usually looks light brow to black in terms of color.

Disadvantages of having mulm in your fish tank

Ammonia, nitrite and nitrate buildup

As we already mentioned above, mulm is organic matter. If left alone, the buildup will eventually break down into ammonia. If you have a cycled tank, the beneficial bacteria will first oxidize ammonia into nitrite and then nitrite into nitrate.

If your tank is not fully cycled, then you could run into issues with mulm. An uncycled aquarium means that the beneficial bacteria could not handle all the ammonia and/or nitrites from the decomposing organic matter. Both ammonia and nitrite are deadly to fish in small traces in the water column.

While nitrates are a lot safer, this nitrogen compound can become harmful over an extended period of time at high concentration. It is best to keep nitrate levels to below 40 ppm in addition to both ammonia and nitrite to 0 ppm. So, if you water test kits are showing concentrations that are above the recommended safe zones, then it may be best to remove the mulm.

If you are looking for a water testing kit, we recommend getting the API Master Test Kit. You can find our review for the product here and purchase it from Amazon with this link. (#CommissionsEarned)

Clog up water flow in your filter

Another issue with mulm is that it can clog your filter, which will end up decreasing the water flow in and out of your aquarium. In the worst case scenario, it can lead to your filter breaking if you let enough mulm build up without doing any cleaning.

Water flow is important as the current prevents any dead zones in your tank. Without any water movement, specific areas in the aquarium could end up having a higher concentration of the aforementioned ammonia, nitrite and/or nitrate.

So, it is best to make sure there is sufficient flow in your filter in order to keep your water clean on all corners of your aquarium.

Prevent plants from getting light

As we mentioned above, mulm can also accumulate on plants. If you don’t clean them off, then the detritus may eventually end up blocking your plants from getting enough light to grow. As a result, you may see some die off, which will then lead into spikes of ammonia, nitrite and/or nitrate if you don’t remove the decomposing parts.

Not only will the dead plants become organic wastes, they will no longer help consume any of the aforementioned nitrogen compounds. So, if you aren’t prepare for it, you could see a huge jump in the concentrations of ammonia, nitrite and/or nitrate.


The last potential issue with having mulm in your fish tank is that it is simply unsightly. Even if your test kit shows that the water column is clean, having brown matter covering the substrate, decor, plants and/or filter makes your aquarium look very dirty.

If you want to show off your tank, then it may be best to remove the mulm beforehand.

Benefits of mulm in aquariums

Breeding ground for beneficial bacteria

A lot of veteran fish keepers love to use mulm from an already cycled tank to kick start the nitrogen cycle of new tanks. This is because the beneficial bacteria love all of the organic wastes that have accumulated on the concentrated substance. Thus, the mulm will hold a high concentration of the bacteria as well.

Nutrients for plants

Your live plants will also love the decomposing matter. The plants will consume the ammonia, nitrates other compounds they viewed as nutrients from the wastes.

For best results, we recommend placing mulm on top of the substrate if you have rooted plants. Of course, you have to make sure that the detritus isn’t potentially blocking off light.

If you have plants that take nutrients from the water column, then it doesn’t matter where you put the mulm as long as the detritus inst blocking off any lights.

Food for fish

Believe it or not, your fish may also eat the mulm. Some species, such as the Cprydoras, Guppies and Plecos like to sift through the decomposing matter to find food. There are even breeders that prefer having mulm in their tanks as they see it as both a food source as well as hiding spots for newborn fry.

Should you keep the mulm in your aquarium?

As long as your water is pristine (you test kit shows that there are 0 ppm of both ammonia and nitrites as well as less than 40 ppm of nitrates), then mulm is relatively harmless in your tank. In fact, it can be quite helpful as it provides food for your beneficial bacteria, plants and fish.

With that said, the organic matter can clog up your filter and lead to a lack of water flow. Furthermore, you also might not like the look of the brown substance being all over your fish tank as it gives the appearance of being filthy. Last but not least, the detritus could lead to spikes of ammonia, nitrites and/or nitrates if you don’t have enough beneficial bacteria to consume the toxins.

Overall, we recommend having some mulm in moderation. In our planted tank, we only remove the mulm if we noticed that the water flow of our filter is slowing down. We also remove mulm if we think that our nitrates level are getting too high because of it.

How to clean mulm from your rocks, decorations, intake sponges and filter media

If you decide to remove mulm in your tanks from the rocks, decorations and intake sponge, we recommending using a plastic bag. This is because lifting the desired object up will cause the detritus to disperse into the surrounding water. The mulm will then end up somewhere else in the tank.

By putting the object into a plastic bag before lifting it up from the water column, you can avoid making a mess. Of course, if you have an external filter, than you can simply take the filter out to clean without having to worry about dirtying the water in your tank.

Once you got all of the dirty objects out of your aquarium, we recommend rinsing them using the water from your tank that you are planning to remove during a water change. This is because tap water, if not treated by an aquarium water conditioner like Seachem’s Safe, contain chemicals that will kill off the beneficial bacteria living on the surfaces on the objects you are trying to clean the mulm off of.

If you kill off enough of your colonies, the nitrogen cycle of your tank will become unbalanced and you have to cycle it again.

How to clean mulm off of the substrate

Getting mulm off of the substrate can be a little bit more complicated. If you have a fine fish net, you can create a whirlwind by creating an upward motion. Hopefully, this will knock lose some of the detritus into the water column where you can then capture using the same net (alternatively, you can also use a different netting).

If you use a tube, you can also suck up the mulm from the substrate. Once you get a siphon going, you can move the tubing to near the substrate to capture the detritus. If you are sucking up the substrate as well, you should reduce the flow of the sipon (you can do so by squeezing the tubing or using a pressure valve).

Alternatively, you can separate the detritus from the substrate later on if you are siphoning both into a bucket or water container.

If you want to vacuum the substrate, we recommend taking a look at the Python No Spill Clean and Fill Aquarium Maintenance System, which you can find on Amazon with this link. (#CommissionsEarned)