I have about 12 plants in my 60 gallon tank. I am planning on adding the following:

6 Albino Corys

3 Black Mollys

1 Male Gourami and 2 female Gourami

3 SAEs (siamese algae eaters)

5 Yo-Yo Loaches

Would that be too high of a bioload? How can I measure the bioload of a tank?


1 Answer 1


Assuming you have an appropriate filter setup and do regular water changes, this is probably fine. My caveat is that AqAdvisor treats the loaches as potentially reaching 10" (25 cm), which would be a bit large. Fishbase says the largest on record was about 6" (15 cm) though; I'm not sure which site has the better handle on that species. (Note that I picked a fairly hefty filter for the AqAdvisor setup, and assumed you have a rectangular tank.)

Bioload isn't one single thing to measure. It actually seems to be more of an aquarist term than scientific. I just did a quick search and found a small number of research papers that use it, generally in the sense of microbial colonization or parasitic infection. (I was wondering if there was a standard unit to quantify it, but that doesn't seem to be the case.) So be aware that it can be used in different contexts with different meanings. I've heard it used to refer to the animals in a system, to the nitrogen waste they produce, to any waste product they produce (even shed hair/feathers/scales), and so on. I think it's useful as a high-level way to think about a tank's ecosystem, but you'll want to be more specific when thinking about the actual pieces of it.

As an overall concept, it lets us think about all the basic components of life needed by the organisms within a system, all the byproducts they create, and the natural cycles of consumption and creation they form. Bioload is, in a sense, a way of thinking about how open (or unbalanced) these cycles are within the tank: the heavier a bioload, the more pressure it places on these cycles. If you've overstocked your system, the bioload is demanding more resources and creating more waste products than you or the system can handle.

We take for granted that some of these cycles are completely open and need constant intervention. For example, the bioload of your system has a certain caloric requirement that it can't produce itself to any real degree, so you fill in that gap by feeding the fish.

Often people say 'bioload' as a shorthand for nitrogen wastes: ammonia (NH3), nitrite (NO2-), and nitrate (NO3-). You should be testing those weekly with a reliable test kit (I like API's), or more often if your system shows signs of having an unstable biofilter. Ammonia and nitrite are toxic to fish in any measurable amount; some nitrate can be tolerated but they should be kept under say 20ppm or so.

In this sense, a bioload is too large when ammonia and nitrite are being produced faster than the biofilter can reduce them to nitrate. It's hard to say "if you have x fish you need y biological filtration" because each species of fish and each model filter is different. But generally it's hard to provide too much biological filtration, so get the biggest filter you can. Plants and water changes will help keep nitrate levels down, and these are really the only way to 'close' this cycle.

Oxygen is another critical cycle. The gas exchange that oxygenates the tank happens at the surface of the water, and you can run into problems if you don't have a lot of water movement to mix the surface layer back down to the middle or bottom of the tank. Plants will help with this too, but you'll still need water motion to get oxygenated water to the entire tank. Odds are you won't have a way to measure how much dissolved oxygen your water has, and as long as your water is moving top-to-bottom and side-to-side you'll probably have enough. You can read more about DO levels at the EPA if you're interested.

There are other cycles as well, like phosphorus and carbon, that you don't need to worry quite so much about. They tend to be at such small scales that normal maintenance easily keeps them closed. These can be more important in heavily planted tanks though.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.