Biological/Biochemical Oxygen Demand (BOD) is a topic we never discuss in the hobby, it refers to largely to the amount of oxygen that aerobic microorganisms use to remove or process waste (Brenniman, 1999) and are directly connected with oxygen saturation and nitrate concentration (Alam et al., 2020).

In a way the hobby talks so little about decomposition focusing on other aspects of nutrient cycling. For a fishkeeper that water changes, siphons and leaves little to no waste or items decaying in the aquarium it might not be of concern.

Botanicals and planted tanks are very popular as of recent with people reaching for some idea of nature they feel they have lost, natural or not. Both of these setups can allow for the trapping of waste where siphoning is not possible or limited. One solution is reduced stocking but definitely keeping fishes adapted for low oxygen saturations is a great solution such as airbreathers.

Decomposition of material such as decaying plants or botanicals involve bacteria, protozoa and other microorganism’s. It’s not just these as an introduction of nutrients but also anything that can be used as a nutrient source for bacteria, I find particularly sugars and carbohydrates. We can split them between aerobic (With oxygen) and anaerobic (without oxygen), anaerobic is another topic here but it does involve the production of other compounds. Just because there is a thick layer of substrate it doesn’t mean it is anaerobic particularly with the presence of plant roots that encourage oxygenation. We also don’t know the rate of either and this will depend on a variety of factors.

These microorganisms are more then capable of competing with fishes for oxygen and the rate will depend on multiple conditions (Nolan, 1996; El-Moghazy & El-Morsy, 2017). Microorganisms can proliferate much faster then fishes so can quickly adapt and increase to those higher nutrient levels.

The issue is that we can barely measure BOD but we can measure oxygen saturation. This means it is difficult to experiment the BOD within any aquarium so we do have to make assumptions.

Most of these are purely assumptions and ideas based on previous knowledge as it’s not so much a topic that the literature will look into. It’s also something fishkeepers certainly need to be thinking about or considering particularly for heavily stocked tanks or fishes who uptake a lot of oxygen.

Temperature, oxygen saturation and decomposition rate

It is a well known effect that as temperature increases oxygen saturation in turn decreases although when thinking about decomposition this increases as microbial decomposers can proliferate at a much faster rate and consume their resources further. This could result in further BOD when there are already low levels (El-Moghazy & El-Morsy, 2017).

Generally it’s better safe then sorry so removing detritus that has built up in the tank and within the filter. Decomposers are probably only providing a benefit maybe for plants but for fishes in many aspects discussed previously they are not of benefit. Any botanicals or high nutrient imputs should be added gradually over time so not to unload a lot of nutrients into the aquarium or when decaying again as much nutrients for these microbes and reducing oxygen saturation.

While we don’t have values and honestly, there is no way of doing that as every aquarium is difficult it’s difficult to predict.

References

Alam, M. S., Han, B., Gregg, A., & Pichtel, J. (2020). Nitrate and biochemical oxygen demand change in a typical Midwest stream in the past two decades. H2Open Journal, 3(1), 519-537.

Brenniman, G. R. (1999). Biochemical oxygen demand. Environmental Geology. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi. org/10.1007/1-4020-4494-1_34.

El-Moghazy, M. M., & El-Morsy, A. M. (2017). Effect of water aquaria changes on growth performance of Nile tilapia Oreochromis niloticus and the relationship between bacterial load and biological oxygen demand. International Journal of Fisheries and Aquatic Studies, 5(3), 341-349.

Nolan, C. (1996). Ventilation rates for carassius auratus during changes in dissolved oxygen.