All you need to know about aquarium bioload and how to control it


What is Bioload?

All the creatures in an aquarium need food and oxygen (among other things) as an input to run their basic life processes.  Since there is an input, there is also an output, in the form of poop and the gases that a creature removes when breathing out. We know that these ‘outputs’ are called waste, and they are called so because they are literally useless and not required in the creature’s body. If not useless, too much build-up of some of this waste can even cause serious health issues. This waste in not needed in the aquarium either, and should be removed. All the waste in a tank, in all its forms together is called bioload, which is short for ‘Biological Load’. A safe amount of bioload for a tank is the amount that the good bacteria can process fast enough to keep the nasties in check. If there is too much for the good bacteria to handle, it will become a problem very quickly. We will see more about bioload and how to reduce it in this article.

Factors that contribute to bioload

Any organic matter that is left lying around in a tank begins to decay. When we say it decays, what happens is that micro-organisms feed on the organic matter and break it down into ammonia, nitrites and nitrates. So, the organic matter is essentially food for the bacteria. The more food that is available to the bacteria, the faster they will multiply. And the faster that happens, they more matter they will break down within a certain time frame.

The organic matter that bacteria feed on includes uneaten fish food, poop, decaying plant matter and dead fish. So if any of these are left around in a tank, the bacteria population will explode, rapidly consuming it. This will cause the ammonia, nitrate and nitrite levels in a tank to rise very, very quickly. This is one of the reasons there should only be a limited number of creatures in a tank, even if there is sufficient space to add some more fish. The more fish, snails, shrimp, etc. there are in a tank, the more they will poop.

How a large bioload affects your aquarium

If the bioload of a tank is too high, there will be a huge growth spurt in the bacteria population, as we have already seen. These bacteria will release a large amount of ammonia, nitrites and nitrates, which are very dangerous to the health of your underwater pets. They will also cause the oxygen content in the water to go down. The oxygen content will go down because of 2 reasons: Firstly, the bacteria will consume oxygen for themselves, and secondly, there is only so much that can be dissolved in the tank water. Since dangerous substances are constantly being released into the water, they take up space, leaving less room for precious oxygen in the water.

Now, while ammonia and nitrates are harmful to your fish and invertebrates, they are a valuable food source for plants. So if they are within a limit, the plants will actually benefit from it and will use it up like free fertiliser! But if there is too much to handle, you could have an algae infestation. Since algae function much like plants, they can grow very quickly, as lots of nutrients will be available to them. You can click here to read on controlling algae outbreaks in a tank.

If there is too much uneaten fish food or decomposing matter like dead fish or rotting plant leaves, you could have a snail infestation on your hands. They breed surprisingly quickly, and can overrun the largest of tanks in under a week. To find out how to get rid of an excessive snail population, click here.

Determining if your tank’s bioload is too high

As important as it is to recognise a higher bioload and control it before it gets out of hand, there is no standard way of measuring it precisely. One might think a better way is to estimate how much a tank’s bioload is going to be before setting it up. How much bioload is generated in a tank depends on a lot of factors, like number of fish, size (mass) of fish, whether the fish is herbivorous, omnivorous or carnivorous, how much waste the fish produces, etc. Due to these factors, it is not possible to accurately predict how fast the bioload is going to increase in a tank.

There are, however, some indicators of a high bioload. The best way to determine a high bioload is to do so proactively, by testing the water regularly using ammonia and nitrite testing kits. These kits can be found at any aquarium store or online. You can test the water for ammonia and nitrites very week. The ideal amount of ammonia and nitrites in a tank should be 0 ppm. Amounts even as low as 0.2 ppm can cause serious health issues, and amounts over 0.2 ppm can cause kill fish within hours, if corrective action is not taken.

Now, the above mentioned test will only give accurate readings in a tank that has already been cycled. Knowing about the nitrogen cycle helps in dealing with bioload too. Click here to read all about it.

Another way to determine if the bioload is too high is that the oxygen content in the water goes down. We have already seen why this happens. When the oxygen is low, the fish will come up to the surface and gasp for air directly from the atmosphere. When you see this, action needs to be taken immediately.

Another indicator of a large bioload, is the population explosion of snails and / or algae, as we have seen.

How to reduce the bioload in an aquarium

Once you find out that the bioload is too high, you need to take these steps to get the level back down. And you need to do them in in the order they are listed.

  1. Test the water for ammonia, nitrites and nitrate levels

The water needs to be tested for ammonia, nitrites and nitrates. Even a level of 0.1 ppm (parts per million) is high, and 0.2 should send alarm bells ringing. The target level should be 0.0 ppm.

2. Do a water change

Water will need to be removed from the tank and replaced with fresh, clean water. Before adding in the new water, be sure to dechlorinate it. The pH level and hardness levels will need to be checked and brought to the levels comfortable to your fish species. The temperature will also need to be brought to the same level as the tank using an aquarium heater. The fish and plants will already be stressed out from the high bioload. Adding in water that is not conditioned properly will only shock them and increase their chances of dying.

3. Increase oxygenation

The amount of oxygen dissolved in the water will need to be increased. You can do so by using an aquarium air pump or filter. Some pumps have a switch to increase the amount of air flow, which can be used in this case. Sometimes, a small valve on the pipe that supplies air to the tank has a valve, which can be opened to allow more air into the tank. While the water added during the water change will contain fresh oxygen, it will only last so long. If there is a power outage or you are unable to increase air flow for some reason, you can increase the oxygen content by changing some more water after an hour or so. Or if you notice the fish gasping for air again.

4. Remove decaying organic matter

Any unnecessary organic matter will need to be removed. These include uneaten food, visible poop floating or lying around in the tank, any dead fish, snails, etc. Rotting leaves and dead plants will also need to be trimmed.

5. Ensure the water parameters are ok

The water in the tank should also be tested for pH and hardness levels. If they are not within the appropriate range for your fish, they should be brought within that range, as they also affect bioload sometimes. If you’re not sure what pH and hardness levels are appropriate for your fish species, you can read about them here. Just click on the profile page for the species that you have.

Avoiding bioload spikes / increasing your tank’s capacity to handle more bioload

Once you’ve brought the ammonia down to a safe level, you want to keep it that way. Here are measures you need to take to avoid bioload spikes in the future

  1. Reduce feeding quantity

The most common reason for a bioload spike is that most of us give too much food to our fish. It may be out of love, or simply because they ask for food all the time. Most species of fish, that are fed flakes, pellets or live food, should only be given what they can finish in under 5 minutes. Other fish that are slow eaters or have specialised diets need more time, like Plecos, Puffer fish, etc

2. Reduce number of aquarium inhabitants

The fewer fish and invertebrates in a tank, the less they will eat and poop. Less poop means the water stays cleaner for longer. Simple!

3. Keep species that are not very messy

Fish like Piranhas (when fed live food) and Puffer fish are extremely messy eaters, who bite off chunks of food and leave the smaller bits behind and ignore them, just leaving them to rot in the water. Plecos take a long time to finish their food, which can cause it to start decaying. Foods with high water content like cucumbers can foul the water surprisingly fast. Avoiding keeping such species will certainly help.

4. Improve the water filtration in the tank

You may not want to follow the above 2 points, and I can understand. Perhaps you’ve kept a fish for a while and have gotten attached to it. Or maybe you’re fond of a species that creates a mess. That’s OK. If you’re ok with reducing the fish in a tank, great! If not, this is what you need to do. You need to increase the water filtration capacity of your tank. There are 3 types of filtration in a tank – mechanical, chemical and biological. The capacity of at least 2 types, mechanical and biological, needs to be increased.

The mechanical filtration can simply be increased by adding a bigger sized filter that can filter more water per hour, or by adding another, smaller filter. The filter (or filters) should be able to filter at least 3 times of the entire volume of tank water every hour. For example, if you have a 50 gallon tank, the filter should be able to process at least 150 gallons of water every hour.

As most filters come equipped with filter media, the biological filtration capacity will also increase automatically. However, while testing the water weekly, if you notice that the ammonia or nitrites are still going up faster than they should, you could add some more surface area in the tank using bio balls or a new dishwashing sponge. The ‘good’ bacteria attach themselves to the filter media, bio balls and sponges, and live there, breaking down any toxic substances. Also, the sponge / bio balls need to be placed in an areas where there is a lot of water flow, ideally in a sump or near the inlet or outlet pipes of a filter.

5. Increase monitoring and maintenance of water and equipment

This step is even more crucial if you have an overstocked tank or messy fish. Checking the water parameters mentioned above more frequently can help notify you of any problems at their initial stages, so you have more time to react and take the necessary steps. The frequency of equipment maintenance, like cleaning filters, heaters, etc. should also be increased.

6. Keep plants

Ammonia and Nitrates, which are harmful to your fish, are actually used up by plants as food. So keeping plants is a sure way to control ammonia and nitrite levels on a daily basis, without you having to do anything!

7. Reduce temperature slightly

Bacteria multiply faster when the water temperature is warmer. So, reducing the temperature while still keeping it at tolerable levels for your fish will slow down their growth rate. Bear in mind that the good bacteria are also bacteria, and their growth rate will slow down too.

Syed Baseeruddin Hyder

I’ve been keeping fish and invertebrates in aquariums for over 5 years. Over the years, I’ve kept more than 15 different species of fish and invertebrates. Through ParadiseInATank.com, I hope to guide new and experienced fish keepers alike with as detailed information as I can get.

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