Post is published, publicly visible.
Translated in Dutch by Elise Rijdes van Soest
Proofread by Joost Abrahams

6.1.2. Uncleaned Mechanical Filtration

Photo of author

Author : David Bogert

Published :

Time To Read :
3 minutes

This concept is where a hobbyist doesn’t clean the mechanical filter. Instead they let the solid feces and uneaten food just naturally decompose. This might have been the origin of the phrase “nitrate factory”. But this practice only increases the nitrate loading by 10% to 15%, not a huge amount. This practice converts a mechanical filter media to a biological filter media.

Through evolution fish have become very adept at absorbing and using virtually all the food they eat. Only 20% to 30% of the nitrogen nutrient value of the food fish eat is defecated by the fish. This is per about 11 books on fish nutrition. When the feces hit the mechanical filter, they are set upon by trillions of fast acting heterotrophic bacteria. This further reduces the nutrient content to about 10% to 15% of the food input. So, the feces collected by a mechanical filter only has about 10% to 15% of the nitrogenous waste that will ultimately end up as nitrate in the aquarium water.

Sciaenochromis fryeri Snow White Sunburst
Sciaenochromis fryeri – Snow White Sunburst

What this means is that if one is trying to hold a nitrate level of 80 ppm, when one does a water change, with uncleaned mechanical filtration the nitrate level might be 90 but with a cleaned mechanical filter the nitrate level might be 75. The nitrate test kits have great difficulty distinguishing between 40 and 80 ppm so the difference is in the noise and relatively unimportant.

This “uncleaned mechanical filter” or “no mechanical filter” is typically a filter where both solid and liquid pollutants are broken down and oxidized to nitrates. This can be the gravel in under-gravel filter, a K1 fluidized bed, a canister filter or a sump. This is a perfectly acceptable means of mechanical filtration which won’t affect the fish.

image of an Aquarium fish Protomelas Fenestratus Fire Blue Lupingu
Protomelas Fenestratus – Fire Blue Lupingu

Most experienced owners of canister filters only clean them every two to six months. These canisters do no mechanical filtration. All the feces and uneaten food that enters these canisters is broken down in the canisters and disappears into the water. These uncleaned canisters add 10% to 20% to the nitrate loading in an aquarium, not of consequence.

Note using a mechanical filtration media as a biological filtration media ONLY works where the conditions are right for it. A large amount of biofiltration, excellent aeration, low food load, and high nitrogen content of the food in some combination can allow this to occur. If the conditions are not right the mechanical media will simply plug up and drop the flow way down.

One YouTube commentator said he use filter floss to give himself crystal clear water and claimed that this was mechanical filtration. He then went on to say he only cleaned out the filter floss every six months or so. In actuality, he was using the filter floss as a biological filter, not as a mechanical filter. His conditions were such that this was possible.

Placidochromis sp Gold Mbamba Bay
Placidochromis sp – Gold Mbamba Bay

Nitrate isn’t Toxic

Then of course there is the simple fact that nitrate isn’t the heavy duty toxin popular mythology says it is. António et. al. 2017 found no long-term effects from a level of 440 ppm nitrate. Monsees et. al. 2016 found the lower long-term damage level for adult cichlids to be 2,220 ppm. Science Direct and Semantics Scholar has several papers available all of which say a level of 440 ppm does no long-term damage to adult aquarium fish.

A recent study exposed young salmon to 440 ppm nitrate for 8 months and they grew just as well as salmon kept at 1/10th that level.  Fathead minnow fry were exposed to 1,575 ppm nitrate for seven days and there was no noticeable effect. This was for FRY! For more information read this article:

5.4. Safe Nitrate Levels