Too many fish in aquarium lead to bioloadYou must have heard the word bioload or biological load if you are an aquarist. When fish are in their natural habitat, the water doesn’t remain in one place and so there is no need to worry about it.

But in an aquarium, there’s no new water unless you add any. Your fish gets to dwell in their own waste, amongst others, and so you need to manage this.

So, what is this waste that is present in the water in your fish tank? Why is it important?

Well, this article will answer all your queries and more.

What is bioload?

The term biological load refers to the totality of the number of organisms living in a demarcated region. In the context of housing fish, this term refers to the totality of the waste that is present in the water column. To put it simply, this term talks about how much waste is present in the water in your aquarium.

To begin with, let’s talk about the source of this load in aquariums. Where does all this waste come from? There are several types of waste that fall under biological waste- decaying plant matter, feces, fish breathing, uneaten food, and other types of waste.

Biological load is largely comprised of waste products that cannot even be seen or spotted by the naked eye. It includes anaerobic (doesn’t use oxygen) and aerobic (uses oxygen) yeast, fungi, and bacteria (paramecium, amoeba, rotifera, etc.). All of these organisms are called infusoria.

Infusoria refers to the aforementioned microorganisms; crustaceans like amphipods, copepods, Cyclops, daphnia; macroscopic invertebrates like crayfish, shrimps, snails; vertebrates like fish; mosses, vascular plants, and algae.

All the aforementioned organisms need food to survive. Similarly, all these organisms excrete waste into the water in your fish tank. Now, the filtration system in your tank only targets a specific portion of the waste released into the water column. The biofilter in your aquarium cannot filter all the different types of waste in your water column.

Most of the inorganic (mineral) products present in the excreta remain suspended in the substrate or water column. They are only removed when you partially or completely change the water in your aquarium or vacuum the substrate in your tank.

What happens to the waste?

As mentioned earlier, the filters in your tank can only remove certain types of waste from the water column. Biofilters can remove waste products containing nitrogen. These waste products get oxidized in two stages.

Nitrogen gets converted to nitrate. The nitrate is released into the water column. The nitrate in the water column is either removed when you change the water in the tank or is used by plants like algae.

So, what happens to the other waste products? A lot of organic compounds remain in the water column. They’re removed only during water changes. Some of these organic compounds have a half-life of a year. Again, some others evaporate, if they’re volatile.

Some of the mineral products may get used up in biological processes occurring in the tank. They are either locked up in the bodies of the organisms (macro and microscopic organisms) in the tank or get destroyed. These essential minerals need to be replenished. When you change the water in the aquarium, the minerals get replenished.

Relevance of biological load in your aquarium

The waste that the living organisms excrete is managed naturally in their natural habitat. New water is always supplied because of water currents and water supply from upstream. However, the situation in a fish tank is very different.

In a fish tank, you have to supply new water. If you don’t perform water changes, the organisms are basically living in that waste-ridden, limited amount of water. Basically, the fish that you’re housing are surviving in an environment full of their own waste. The quantity of that waste is determined by the strength and efficiency of your filtration system.

This is where biological load becomes relevant. If your aquarium bioload is very high, the filter in your fish tank won’t be able to work efficiently. The types of fish, plants, and other creatures you can house in your tank are determined by the type and efficacy of your filtration system.

If you have more organisms than your filter can handle, the quantity of waste in your aquarium will increase. There will be an excess of nitrites, ammonia, and nitrates in the water. This will put too much pressure on your filter.

Your filter has to perform a lot of functions. It has to trap the waste products in your tank. It also has to trap enough bacteria so that the nitrites and ammonia get processed. Excessive pressure on your filter means poor water quality, leading to algae blossoms, disease outbreaks, and even death.

How to handle bioload increase

So, what can you do to handle any increase in the waste concentration of your aquarium?

Here are some strategies to handle any increase in biological waste levels in your tank:

– Regulate the amount of food you’re giving to your fish

Overfeeding your fish is not a good idea. Large quantities of food or very frequent feeding sessions can increase the biological load in your tank. If your fish eat pellets, live foods, or flakes, just give them portions that they can finish in under 5 minutes.

– Avoid keeping fish species that are messy

Avoid housing fish that eat live food and are messy eaters. Puffers and Piranhas fall in this category. These fish species have a tendency of biting off chunks of food. They leave smaller bits to rot in the water.

– Limit the number of organisms you keep in your fish tank

Every living organism releases waste products into the water. So, the fewer invertebrates and fish you keep in the aquarium, the lower will be the waste concentration of the water in the tank. Try to keep a limited number of organisms in the aquarium, especially if you have a small fish tank.

– Monitor the filters and the water in your aquarium more frequently

If you were in the habit of checking the water quality of your aquarium and filters say, once a week, increase the frequency. Prevention is the best way to deal with this issue. Check the heaters, filters, water parameters, etc, more often. In this way, you can identify a problem in its initial stage.

– Regulate the water temperature in the aquarium

Bacteria are able to multiply in higher-temperature environments. So, try and reduce the water temperature in your aquarium. Keep it at a level that will allow your fish to thrive as well as keep the bacteria levels low.

– A better water filtration system

If you can increase the filtration capacity of your tank, it’s one of the best ways to deal with this problem. Mechanical filters, biological filters, and chemical filters are the three main types of filters. You have to increase the water filtration capacity of at least 2 filter systems- biological and mechanical.

– More plants

Nitrates and ammonia are both very harmful to fish. However, if you keep more plants in your tank, these plants can use up the ammonia and nitrates. So, keeping more plants is a natural way of regulating the ammonia and nitrate levels in your aquarium!

How to reduce aquarium bioload

If you’ve discovered that your reef tank bioload is very high, apply the following strategies to reduce it:

– Perform a water change

Remove the dirty water from your tank. Replace it with clean water. You have to basically perform a complete water change. However, before you add the fresh water to the tank, you must dechlorinate it first.

Also, the hardness level, pH, and temperature of the water have to be regulated to what your fish species need. Performing a total water change can cause a lot of stress to your fish. So, you need to do it properly, keeping all the aforementioned details in mind.

– Test the ammonia, nitrate, and nitrite levels of the water in your aquarium

You need to test the tank water for ammonia, nitrate, and nitrite levels. A safe ammonia level is nothing more than 0 ppm (parts per million). Below 25 ppm is the preferable limit for nitrate, while the safe limit for nitrite is 0.75 ppm or lesser.

– Remove all decaying organic matter

Remove organic matter like feces, uneaten food, any dead organisms, etc, from your tank. Trim any rotting plants and rotting leaves too. This goes a long way in decreasing the load.

– Increase oxygenation

To boost oxygenation, you can add an aquarium air pump or an air filter to your tank. You can get a pump with a switch to increase the aeration in your tank. Alternatively, if you have a pipe to aerate your tank, check to see if it has a small valve. You can remove this valve to increase the aeration in your aquarium.

– Check water parameters

Last but not least, you have to check certain water parameters. Make sure that the water temperature, hardness, pH, and other parameters suit the needs of your fish. Proper parameters help to alleviate issues.


Let’s do a recap of some of the important facts mentioned in this article about bioload in small fish tank:

  • Fish in home aquarium producing bioloadBiological load refers to the totality of the number of organisms living in a demarcated region.
  • Decaying plant matter, feces, fish breathing, uneaten food, and other types of waste are all biological load.
  • The quantity of this waste is determined by the strength and efficiency of your filtration system.
  • Poor water quality can lead to algae blossoms, disease outbreaks, and even death.
  • To handle increased waste concentration, keeping more plants, reducing the quantity and frequency of feeding, keeping a better water filtration system are some key strategies.
  • Perform a complete water change to reduce the waste concentration in your aquarium.

If you’re an aquarium hobbyist, acquainting yourself with this bioload issue, its relevance, and the strategies to deal with this issue are all very important.

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