What is Mulm or Detritus in Aquariums and Ponds? | Real Aquatics

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What is Mulm or Detritus?

Mulm, or 'detritus', is the organic debris that builds up in and on the aquarium substrate or the bottom of a pond. Mulm is usually the unattractive mushy brown or black material that settles on the substrate (sand or gravel) of a tank.

Detritus is usually made up of dead organic matter such as fragments of dead organisms, plants, uneaten food or fish waste (fish poo!). You also find this substance in your filter media such as sponges foam, bio balls and ceramic rings - anything porous.

If left alone, the buildup of detritus in your aquarium or pond will eventually break down into ammonia, so you do not want it to build up. If you have a cycled fish tank, the beneficial bacteria will first oxidize ammonia into nitrite and then nitrite into nitrate. Too much ammonia and nitrite can be deadly to fish, even in small traces in the water.

How do I remove Mulm?
Mulm can be easily hoovered up in a fish tank using an aquarium siphon, and pond sludge and mulm can be reduced by using a pond vacuum regularly.

Detritus tends to pile up at the bottom of a fish tank in low flow areas, such as corners or behind decorations rocks and plants. If you have baby fish (fry) or shrimp in the tank, be careful not to hoover them up when gravel siphoning. Pond detritus often tends to be more widespread, as it can often consist of heaver plant matter, leaves, treated algae (sludge) and small twigs.

Do snails eat Mulm?
Most aquatic snails will eat at least some of the mulm, especially if it is made up of plant matter or decomposing food or organics. In ponds the Ramshorn Snails is a popular addition for this reason, and in aquariums the nerite or apple snail will also do the task. Unfortunately, its also a good source of food for pest snails and detritus worms.

Do Plecos, Catfish or other fish eat Mulm?
Your fish, believe it or not, will consume at least parts of the Mulm. Some species such as Corydoras, Guppies, and Plec's like to sift through the mulm in search of food. Mulm, or Detritus, is preferred by some breeders since it serves as both a food source and a hiding place for newborn fry - although it is not often left to accumulate to an excessive degree.

Do I need to remove mulm, or detritus from my fish tank?
Detritus is dead organic matter such as fragments of dead organisms or fish waste that can collect on the bottom of a fish tank. If left alone, the accumulation of these elements can cause ammonia, nitrates and phosphates to rise, leading to algae outbreaks. Use a syphon cleaner to perform this task.