Last update: November 12, 2022

What the Heck Is Bioload and Why Is It Important to Your Aquarium?

When reading online, it’s likely you have stumbled across the word bioload.

Confused, you likely Googled “What is bioload?”

Well, at the time of writing this, there wasn’t an article that answered this question.

Seeing this, I decided to create one myself.

So, let’s jump right into it…

What is bioload?

Bioload is short for biological load, but unless you read scientific papers, it is unlikely you will ever come across it written like this. In over 30 years of fish keeping, I have only ever heard it referred to as bioload.

So, what is bioload?

To put it simply, bioload refers to all of the waste in the water column, which is the water inside your tank.[1]

Where does this waste come from?

Fish, shrimp, snails, plants, corals, even bacteria and other microorganisms you can’t see…

If it’s alive, it’s going to add waste to your aquarium.

A couple of common types of waste include:

  • Poop
  • Uneaten food
  • Decaying plant matter
  • Fish breathing
  • Any other type of waste

All this waste is collectively referred to as bioload.

Also learn everything you need to know about bio balls.

How does bioload relate to your aquarium?

In the natural environment, new water is constantly entering the fish’s habitat. Whether that’s through water flowing from upstream or ocean currents, waste does not have an opportunity to accumulate.

Now, let’s come back to your aquarium…

Your aquarium is filled with a small amount of water – and that’s all it gets. Except for a water change every now and again, no new water enters the tank.

So, a filter is used to recycle the water, cleaning it before it returns to the tank.

In a perfect world, a filter would clean 100% of the water – returning it to the state it was in when it first entered the tank.

But as we all know, neither the world nor filters are perfect.

In an aquarium setting, your fish sit in a pool of their own waste.

And, just how much waste is determined by your filter.

This is where bioload comes into play.

If the bioload in your tank is too high, your filter will be unable to keep up with the amount of waste.

The capabilities of the filter are what limit the amount of fish and other live plants and creatures that you can keep in your aquarium.

If you have brushed up on the nitrogen cycle, you will know that waste leads to ammonia, nitrites and nitrates being released into your aquarium.

Your filter needs to not only trap waste but also hold enough bacteria to process ammonia and nitrites.

If your filter can’t effectively clean the water in your tank, then the water quality is going to drop, causing complications like disease, algae blooms and even death.

RelatedHow to cycle aquarium faster?

How do you determine the bioload of your aquarium?

Here is where it gets a little more complicated.

The bioload varies from fish to fish.

Generally speaking…

  • Large-sized fish have a big bioload.
  • Small-sized fish have a tiny bioload.

However, this is also impacted by…

  • Whether the fish is a meat eater or plant eater,
  • How efficient the fish’s digestive system is, and
  • How much the fish eats.

Of course, the more fish in the tank, the higher the bioload.

Let’s use a 30-gallon tank as an example…

A filter that works for 30 zebra danios will not be able to keep up with 10 gold fish – the gold fish eat and poop a whole lot more than the danios.

This brings me to the frustrating thing about bioload…

There is currently no effective way to calculate the bioload of each fish.

There isn’t even a standard unit of bioload measurement.

Yep, you read that right.

You see, bioload is more of an unofficial term used by aquarists when describing how to stock a tank.

When it comes to determining bioload, it comes down to experience.

RelatedWhy are your fish dying?

In some instances, identifying a high bioload will be obvious – you can’t keep 10 goldfish in a 10-gallon tank.

But when it comes to mixing multiple species of fish in the same tank, the answer will vary according the person you are speaking to.

While there are calculators available to help you determine the bioload of fish before buying them, these are often criticized for their accuracy and lack of data.[1]

I hate to break it to you, but determining bioload often comes down to trial and error.

Fortunately, there are many experts who have already done this trial and error themselves…

There are plenty of people in online forums, hobbyist groups and even the staff at your local fish store who will be able to point you in the right direction regarding an appropriate bioload for your aquarium.

How to test the bioload in your aquarium

The end goal is to avoid having a bioload that is too high for your tank…

If the bioload is creating more waste than your system can handle, then you will throw the nitrogen cycle out of balance, leading to constant problems in your aquarium.

You will know that the aquarium bioload is too high when ammonia and nitrites are being produced faster than the bacteria in your filter can convert them to nitrates.

Fortunately, this is relatively easy to test for, since ammonia and nitrite levels should be 0 ppm.

Any indications of ammonia or nitrites – After your tank has cycled, of course – could clue you in that the bioload is too large for your aquarium.

If this is the case, you have two options…

  • Get a larger filter or
  • Get rid of some fish.

I know it can be painful to say goodbye to your pets, but it will be worth it in the long run. The remaining fish will be much healthier because of it.

Your only other choice is a larger filtration system. This can be quite an expensive option depending on the size of filter required.

At the end of the day, you should consider your tank’s biological load in relation to all the other cycles that are occurring inside your tank.

Doing so will ensure that you are putting all the puzzle pieces together, allowing you to raise happy and healthy fish.

Do you have any more advice to share on bioloads? Let me know in the comments below!

Ian Sterling

I've been keeping fish for over 30 years and currently have 4 different aquariums – it's an addiction. I'm here to teach you everything there is to know about fishkeeping.

I also use this site as an excuse to spend lots of money on testing and reviewing different aquarium products! You can find my reviews here.

Comments (6)

I am a ni g to have a fish tank that is 11.5 inches wide. The ledge is 12.5 inches wide
It will be 35 inches tall and 40 inches long.
I want to have some colorful fish and some plants both plastic and real plus some buildings
Maybe some snails or crabs
I really want it colorful for my grand children to enjoy when they come over.
Please help me. I know nothing about this subject other than what you have taught me in this article
New at this

Hi Rosie,

There is plenty of information to be found online one whatever you need to know. Just google it one step at a time. Before you can add fish, you need to cycle your aquarium, which can take some time.

After that, check out for stocking suggestions. The 1 inch per gallon rule isn’t accurate at all.

Do you think a 30% biweekly water change is good for a betta (no plants or other creatures) in a 10 gallon tank?

Hi Mike,

You do water changes as per your test kit results. It will indicate if you need to do water changes at a greater frequency.

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