The Science of “Detritus”
In the eyes of most hobbyists “detritus” is any “brown gunk” in an aquarium. Other words used to describe this material is mulm, organic matter, sludge and debris.
The subject of biofiltration in an aquarium is very dependent on the subject of “detritus”. In biology, detritus is particulate organic material, normally considered “dead” but actually teeming with microscopic life.
Note that you will come across very authoritative-sounding information trying to define mulm and detritus as two different things, “mulm” being “alive” and “detritus” being “dead”. In fact there is no such thing as “dead” brown gunk in an aquarium. It is ALL alive to a much greater extent than realized.
In the aquarium there are many types of detritus:
- “Bad”, undesirable detritus
- brown scum on the surface of the mechanical filter (largely feces)
- brown mulm above the substrate which contacts the fish
- Benign detritus
- “brown gunk” in the interstices of the substrate where there is NOT decent water flow.
- sometimes brown mulm above the substrate where the fish do not contact it
- Desirable Detritus
- “brown gunk” in the interstices of the biomedia in the filter where there IS decent water flow.
- sometimes brown mulm above the substrate where the fish do not contact it
In general, detritus which doesn’t contact the fish is beneficial, acting to oxidize dissolved organic compounds and ammonia to relatively harmless gases and nitrate with a whole host of living organisms.
But because detritus has bacteria in it, detritus which contacts fish is generally detrimental. The bacteria require the fish to give a sizable portion of its immune system over to fighting the bacteria, even if that bacteria is harmless. This leaves less immune system for fighting off pathogens.
Brown Mulm Above the Substrate
Some tanks get a floating cloud of brown material exiting in clumps just above the substrate. It is called “mulm”. This mulm is only of concern if one has bottom feeders like corydoras or bottom swimmers like goldfish.
This mulm contains a lot of bacteria. The bacteria will create big health problems with the likes of corydoras and clown loaches. Loaches swimming through mulm CAN get columnaris/epistylis and corydoras swimming through mulm CAN lose their whiskers to bacteria. This whisker loss is often incorrectly blamed on “sharp substrate”.
Mulm generally has several causes:
- The aquarium is over stocked
- The aquarium has poor water circulation
- The aquarium has poor aeration
- The aquarium is being over fed
Most of the time a combination of several of these things is present.
The food variable is probably the most common cause of too much mulm. Fish do not need much in the way of food. They only need 1% to 2% of their body weight in dry fish food. This is typically the volume of two eyeballs per fish per day.
Note that if one has a breeding tank with small fry in the tank this mulm CAN becomes desirable. Fry do best with a constant source of food. The mulm produces small organisms the fry can eat (infusoria). The fry seem to be immune to the bacteria in the mulm.
Aquaculture operations which have very heavy stocking of fish in a pond need to remove the mulm on the bottom of the pond at least once a year. The brown mulm will start giving the fish all sorts of health problems. But what about farm ponds? Farm ponds have a light stocking of fish and a heavy brown mulm on the bottom. This mulm is healthy and doesn’t harm the fish. Indeed it is where most of the beneficial bacteria reside in a pond.
Some hobbyists have lightly stocked, unfiltered aquariums with heavy mulm accumulations (one form of the “natural” aquarium). The mulm acts like a filter. These lightly stocked well aerated aquariums can have healthy fish. It just all depends on stocking and aeration. Note that every “natural” aquarium I’ve ever seen with heavy mulm … well … how do I say this …. looks like crap to me. There are invariably small particles of mulm floating around the tanks which I dislike intensely. And I just don’t like the “dirty look”. But to some this is beautiful. “To each his own”.
Getting Good Circulation
Mulm is reduced by good water circulation. If one has poor water circulation one can optimize the circulation by adding two small bladed circulation pumps (“wavemakers” or “powerheads”) per aquarium. Add one pump at the upper back side of the aquarium. Aim this at the surface in such a way as to maximize the choppy water surface area in order to maximize aeration (this is typically about 30 degrees upward).
Then add, at the side opposite the first pump, a bladed circulation pump at the back at a middle height, aimed at 30 degree downward angle to the middle front of the aquarium. This pump keeps the front substrate free of feces without being too obtrusive. These two pumps give a good circular flow in the aquarium.
Getting Good Aeration
Mulm is also reduced or eliminated by aeration.There are TWO good ways to aerate any aquarium. Either a “choppy’ surface OR air stones.
- Aeration at the surface of an aquarium is a matter of getting a surface that is full of choppy waves. These choppy waves are “turbulent” flow, which aerates the water many times better than smooth or “lamellar” flow. Choppy surface water can be from a fan bladed circulation pump (a “wavemaker”) aimed at the surface or from the outlet of a filter pump positioned just below the surface and aimed at the surface.
- Air bubbles rising from an airstone through the water ALSO do a great job of creating turbulent flow air/water interface, contrary to popular myth. They do need a decent sized air pump in order to do a good job. One wants a LOT of bubbles.
Brown Scum on the Surface of the Mechanical Filter
The brown scum on the surface of a mechanical filter is largely fish feces and uneaten food. This brown scum is normally cleaned off once a week (or the filter media is changed out). This brown scum is actually much more benign than many think. It only contributes a small amount to the pollution in the tank, typically ten to twenty percent of the nitrate is from the mechanical filtration scum.
All the author’s aquariums (18 last count) have no mechanical filtration (and a huge amount of biofiltration) and do just fine. If one has a limited amount of biofiltration (like a hang on back or a sponge filter) this detritus becomes more detrimental and needs to be cleaned regularly.
“Brown Gunk” in the Interstices of the Substrate
This brown gunk is relatively benign and has some slight positive benefit. It can be removed by deep vacuuming every month or so (unless it is an under-gravel filter) but it really isn’t necessary. Because the flow is very slow in the substrate this brown gunk does NOT do any significant biofiltration, contrary to popular mythology. It also does NOT go anaerobic and produce poisonous gas. It also does not decompose rapidly and release heavy spikes of ammonia, nitrite or nitrate. It is not a “nitrate factory”. Most of the time it basically just sits there, SLOWLY processing and decomposing dissolved organic compounds and ammonia into carbon dioxide and nitrate.
But like all things there can be instances where this changes. If a beginner puts four large Oscars in a fifty gallon (189 liter) aquarium and feeds them “what they can consume in five minutes three times a day” because that is what the directions on the fish food box say, one can have a problem in any substrate. The brown gunk in the substrate can go hypoxic and release bacterial toxins. The toxins stress out and irritate the fish.
“Brown Gunk” in the Interstices of the Filter Media
This is the “good” detritus. It is essential for removal of ammonia. What is missed is that this brown gunk is even more important for keeping bacteria in the water column at bay.
Cloudy water and/or ammonia spikes have the same root cause. The root cause is that the filter doesn’t have enough dirty slimy brown “gunk” in it. The brown “gunk” in a filter is floc with a biofilm of beneficial bacteria. This brown “gunk” is what you find in the media in filters. It takes many months to form. “Beneficial bacteria ain’t pretty”.
There are at least three reasons for ammonia spikes or cloudy water:
- The filter is less than 3 months old and hasn’t had time to form a large amount of brown “gunk”, i.e. beneficial bacteria
- The filter is a hang-on-back with insufficient surface area to form a lot of brown “gunk”, i.e. beneficial bacteria
- In our “cleanliness is next to godliness” world, everyone keeps cleaning away this brown “gunk” because it’s “obviously fish poop”. And they are cleaning away their beneficial bacteria.
This concept that beneficial bacteria are in the brown gunk in filters has its basis in several factors:
- Diana Walstad gaat in haar boek "The Ecology of the Planted Aquarium" uitgebreid in op de werking van nuttige bacteriën. Ze gaat uitgebreid in op de opbouw van biofilms en bioflokken met een groot aantal referenties.
- Bij het starten van een aquarium is iedereen het erover eens dat het beste "zaadje" is om een vies bestaand sponsfilter uit te knijpen in wat water. De resulterende bruine soep kan binnen enkele dagen een ingedraaid aquarium opleveren. Dit is dus duidelijk de plek waar de nuttige bacteriën zich bevinden.
- K1 fluidized beds work great at keeping water crystal-clear and minimizing ammonia and nitrite readings. They only do that when they have been cycling for two to four months and build up a brown filmy mass inside the K1 media. This filmy biofloc is where the beneficial bacteria reside.
- Frequent complaints on social media say things like “My ammonia spiked” or “my water is cloudy”. Ask a simple question “Did you just clean or change your filter?”. And easily 75% of the time the answer is yes. This is just one of the most common mistakes people make.
Van de Poret Foam-leverancier (Swiss Tropicals):
Het bruine filterslib in een filter is voor het grootste deel levend en niet zomaar afval. Het verwijderen van deze modder doet meer kwaad dan goed. Het doel van de filtermedia is niet om deeltjes uit het water te filteren zoals vaak wordt aangenomen. De media dienen als leefgebied voor een breed scala aan micro-organismen, waaronder bacteriën, archaea, wormen, ciliaten, flagellaten en vele anderen. Deze micro-organismen leven in een gemeenschap die gebaseerd is op biofilms. De biofilms worden gecreëerd door bacteriën die extracellulaire polymere substantie (EPS) afscheiden, die vaak “slijm” wordt genoemd. De gemeenschap vormt een bioreactor die het afval verwerkt en omzet in voedsel en energie voor haar leden, en uiteindelijk in organische of anorganische producten die vervolgens door planten worden gebruikt, verdampen of worden verwijderd door waterverversing. Het kost veel tijd om deze “filtergemeenschap” op te zetten; daarom is het erg belangrijk om het niet te verstoren tenzij het absoluut noodzakelijk is.
Swiss Tropicals
This is probably the best write-up from an aquarium product supplier on any subject. It is 100% correct. The organisms in brown floc eat dissolved organic compounds, bacteria, fish pathogens like ich trophonts and each other in a complex web. It is a miniature sewage treatment plant and it works very well if it is just left alone to do its thing.
This brown filter “gunk” has another benefit. If one gets some ich in the tank this “brown gunk” develops a host of tiny organisms which literally eat the infectious stage of the ich, the trophont. In this manner the ich breakout is controlled and the fish survive. If one does not have a mature filter, ich can rapidly kill fish.
De wetenschap van filtratie in meer detail
There are some aquarium hobbyists who are interested in delving deep into the science and the calculations behind all aspects of filtration and detritus. For those who are so inclined the following is pertinent:
Dan is er de wetenschap achter het indraaien voor degenen die nieuwsgierig zijn. Deze wetenschap wordt beoordeeld in de volgende links:
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