Understanding Biological Surface Area in Aquaponics

Growing with aquaponics can be a fantastic way to experience higher yields, better efficiency and healthier plants.

To function well, aquaponic systems depend on a complex and robust ecosystem to cycle nutrients and create balance between organisms and their environment.

One aspect of this type of production method that often gets overlooked is biological surface area (BSA) in aquaponics.

This post is to help you better understand the importance of biological and specific surface area to produce higher yields and fewer frustrating mistakes!

What is Biological Surface Area?

To start, biological surface area (BSA) is the amount of surface area inside your system that on which microbes can live. BSA is very important in aquaponic systems because these microbes are the engines of a healthy aquaponics system.

Microbes oxidize ammonia, assist in nitrification and mineralize materials like iron in order to foster healthy plant growth and a healthy system overall.

Measuring Biological Surface Area

We typically measure BSA in the total number of square feet per system.

To fully grasp this measurement, we’ll also need to understand how much specific surface area (SSA) is our system. SSA is measured as the number of square feet per cubic foot (ft2/ft3).

This is the amount of square feet there are inside of the volume of media you’re using.

Once we have calculated the specific surface area, all we have to do is multiply the SSA by the VOLUME of the grow beds or ZipGrow Towers to get the Biological Surface Area.


Say you have a 700 square foot greenhouse. Leaving 30% space for access and maintanence, you’re left with 500 square feet of growing space. Let’s look at two growing systems within that space: one using ZipGrow Towers with Matrix Media (SSA 290 square feet per cubic foot) and one using media beds with pea gravel (SSA 85 square feet per cubic foot).

You’ll see that in terms of BSA, the two systems are a bit different. Another factor that you have to keep in mind, however, is how much of growing space you have per square foot of BSA. 

Why Understanding Biological Surface Area is Important

Figuring out how much BSA is in your system will help you to understand whether or not your fish are understocked or overstocked and help you make the adjustments necessary for a efficient, effective growing system.

To give you an idea of how much BSA/SSA is in various media types, I’ll turn it over to Dr. Nate Storey’s research on the matter.

From Storey, 2012:

Table 2.01 Specific surface area comparisons for different substrates.

Particle SizeSpecific Surface Area
Media Typeinchesmmft2 ft-3m-2m-3Void Ratio (%)Hydraulic Conductivity (m/d)
Medium Sand0.123270886401
Pea Gravel0.5714.58528028104
Large Rock4102123948106
Plastic biofilter media1258528090107
Plastic biofilter media2504815793108
Plastic biofilter media3.5893812595108
ZipGrow Matrix MediaN/AN/A29096091107*

* Estimated to be approximately that of small diameter plastic biofilter mediaAs you can see, different medias have drastically different biological surface areas.

“These studies are especially relevant to this research, and especially the design phase of Tower development, during which the properties of the media used had to be closely defined. Deciding on the media type was difficult and literature detailing the inverse relationship between particle size and Specific Surface Area (SSA in m2 m-3) was useful. This is due to the relationship between percolation and SSA that is a feature of most aggregates. As particle size gets smaller, specific surface area for that media type increases, that is to say, the surface area to volume ratio increases, i.e.:

– Medium sand (3 mm diameter), SSA= 886 m2 m-3;

– Pea gravel (14.5 mm diameter), SSA=280 m2 m-3;

– Medium gravel (25 mm diameter), SSA=69 m2 m-3;

– Large gravel (102 mm diameter), SSA=39 m2 m-3; (Crites, et al., 2006).

It should be noted that values in the literature can be somewhat contradictory depending on the source. This is primarily due to differences in measurement and classification standards. What these values will show however, regardless of technique, is that smaller particles are better suited for integration into systems where high SSA values are important.

Unfortunately, the reality is that these small particles trap solids much more efficiently and rapidly foul with accumulated biosolids, leading to anerobic conditions and lower dissolved oxygen (DO) concentrations that negate the benefits of small particle size. This low hydraulic conductivity and small pore size (low void space/void fraction) makes small-particle media inappropriate for most biologically active systems with active cycling. To avoid this problem, larger particle sizes are commonly used (17 mm crushed granite or ¾ inch crushed granite) having higher void ratios (and resulting high hydraulic conductivity) so that solids impact percolation less. However, even though these crushed aggregates have significantly higher SSA than non-angular and non-crushed aggregates, SSA is still comparatively low, resulting in reduced overall system Biological Surface Area (BSA or total surface area of system measured in m2).”

Calculating your BSA

Remember: as an absolute minimum, your system needs at least:

2.5 ft2 of BSA/gallon of water (at low stocking densities and low feeding rates)

For a healthier system, we would recommend:

10 ft2/gallon of water OR 100 ft2/pound of fish.


If you’re stocking fish at 1 pound per 10 gallons, for every pound of fish, you’ll need 25 ft2 of BSA – This will be the amount you’ll need for adequate waste and ammonia processing.

Does the Age of my System Matter?


Generally speaking, older systems are going to be much more efficient at processing waste (i.e. the microbial communities inhabiting older systems are much more established, stable and able to operate more effectively as a result).

Younger systems (see: newer/less mature systems), you’ll need more BSA right away to help in the nitrification process.

** IMPORTANT: If you haven’t properly cycled your system, it doesn’t matter how much biological surface area you have.**

Remember: A truly healthy AP system requires as much Biological Surface Area as possible – BSA is the horsepower of your aquaponics system!

ZipGrow Towers & High Specific Surface Area

If you noticed in the table above, ZipGrow Towers have a very high SSA, BSA and void ratio.

The reason for this is that they were designed this way!

As you see in the table, our Towers and Matrix Media have 290 square feet of specific surface area per cubic foot of our media.

Our media fibers provide a ton of surface area for our microbes to hang out on and keep our system healthy.

The high SSA, in combination with a void ratio of 91%, which allows water and solids to flow through our Towers easily, creates a productive powerhouse in our aquaponics system. (Don’t forget the light weight and ease of transport/maintenance!)

This media and ZipGrow Towers are available to anyone on our online store.

             Particle SizeSpecific Surface Area
Media TypeInchesmmft2 ft-3m-2m-3Void RatioHydraulic Conductivy (m/d)
ZipGrow Matrix MediaN/AN/A29096091107*

Find more examples and equations in our video on biological surface area in aquaponics.

Planning a Farm?

Upstart University is the online training program to learning everything you need to become an Upstart Farmer. Course videos, written content, quizzes, and assignments are designed to empower and equip you.

Welcome To ZipGrow Blog

Our goal is to produce high-quality informative content to highlight the power of local food, small business, and how delicious sustainable food can have in our communities and in our world.

Recent Posts

Check Us Out On YouTube!

Enjoyed This Post? Sign up for our Newsletter

Subscribe today to stay updated! Only the good stuff, we promise.