[DISCUSSION] Bioload of an Aquarium: How much is too much?

Singapore Reef Club · January 25, 2021

Bioload of an Aquarium: How much is too much?

How many fishes can I keep in my aquarium? Finding the limit of the bioload of our reef aquarium is a frequently debated question. On one hand, we don’t want to pack our fish tank like canned sardines. On the other hand, it’s nevertheless good to understand the limitation of our little glass box. If you ask our friend google, it’s easy to find suggestions such as “1 inch fish per gallon of water“. For an absolute beginner, it’s an easy to understand guideline (with a lot of caveats). However, for you, my dear reader, you deserve to understand better. I will not discuss how many fishes (or corals) can be kept in a reef tank. Rather, I would like to discuss about what is stopping you from keeping more and the chemistry/reasons behind it. This article touches on many topics, so buckle up!

Challenges of High Bioload

To answer the question about bioload, we need to understand the negative impact of an overly heavy bioload. When the bioload is too high, there are two critical challenges:

The social dynamic of the reef fishes is compromised. This manifests as aggression, stress or neurotic behaviors. If there are too many fishes in the aquarium, the fishes are more likely to compete for food, hiding place and dominance. Therefore, undesirable behaviors are more likely to occur. You can read more about fish aggression in this article.
The chemical balances of the reef water is compromised. In a closed system such as our reef aquarium, reefers make use of a variety of techniques to counteract the impact of shifting chemical balance. We strive to keep components (nutrients, reef-building elements, dissolved gases, etc.) of reef water within a range of acceptable values. When the change in water parameters outpaces the coping ability of the reef system, the aquarium may spiral out of control.

A lot of unseen chemicals exist in this clear water

The breakdown of the social dynamics and the water chemistry can cause the fishes to experience a higher level of stress. The affected fishes may turn aggressive or becomes withdrawn. The immune system of the fish becomes compromised, and the fish is more susceptible to infections due to overcrowding.

This article, we will focus on the chemical balance of our aquariums.

Chemical Balances

Unlike in the open ocean reef, keeping a boxed aquarium is an enclosed system where Reef water is the immediate environment of marine lives we keep. Therefore the “quality” of the reef water in an enclosed marine tank is paramount for the success of the aquarium depending on our filtration systems and caretaking routine. When fishes and invertebrates eat, breath and releases wastes, the chemical balance of the water changes. We will look into three categories in this article.

1) Dissolved Gasses

Hold your breath, count to hundred. It’s difficult isn’t it. It’s the same for the fishes in our aquarium.

Concentration of dissolved gases in Seawater. We are only interested in the top 20 m or so.

Respiration is the process of producing energy from food. Vast majority of living things relies on aerobic respiration for energy needs. When a fish goes about its daily business, it extracts oxygen from water, and release carbon dioxide at the same time.

When the dissolved oxygen level in the water drops to a very low level, fishes will have labored breathing. In the most extreme of cases, fishes die due to asphyxiation. I vividly remember my trip to Tibetan plateau. Over there, the air is very thin at high altitudes. and the oxygen concentration is very low. For the first few days, I have to be very conscious about breathing. Even then, I experience headaches and fatigue due to lack of oxygen. I guess fishes in an aquarium with low dissolved oxygen would feel the same – not at all pleasant moment to be in.

On the other hand, the released carbon dioxide presents a different challenge for the aquarist. Dissolved carbon dioxide ( C02 ) is in a delicate balance with bicarbonate and carbonate ions (alkalinity). The ratio of the three determines the pH of the aquarium water. Therefore when there is too much carbon dioxide in water, the pH level drops. This can impact the physiology of marine lives.

2) Maintaining Gas Balance

So how do aquarists deal with these two parameters in check? Easy. We promote gas exchange. The law of nature dictates that matters will move from more concentrated area to less concentrated area. This is called diffusion. When we create a water-air boundary, diffusion can take place. Oxygen molecules enters water from air, while the carbon dioxide molecules does the opposite. If there is sufficient air-water boundary, the dissolved gasses will take care of itself.

Simplified diagram representing gas exchange.

A tank of water have very limited surface for gas exchange. Fortunately, we have some ingenious tricks to massively improve the situation.

Water flowing down the overflow weir increases gas exchange.
Protein skimmers creates numerous microbubbles. As a result, this vastly increase the air-water boundary.
Good circulation using wave maker etc breaks the water surface and improve gas exchange.
Keeping an algae refugium or scrubber take advantage of photosynthesis. This produces oxygen while consume carbon dioxide in the system.

3) Nutrient Balance

Nutrient in reef is a complex topic. Perhaps we can discuss about this in detail later. For now, I would like to offer a simple approach towards understanding nutrients.

Unsurprisingly, Reef Nutrition adds … nutrition

Nutrients in the Reef

What is precious and often fought over in the nature become an evil in our reef aquarium. Elements such as Nitrogen, Phosphorus and Carbon are resources on the natural reef. Fish, corals and invertebrates evolved to take advantage of what little nutrients available on the reef. For example, coral produces mucus, which is capable of collecting phosphates from surrounding water. As a result, the phosphate level in coral mucus can be more than 100 times of surrounding water. For this article, I will not discuss about what the ideal level of N:P:C is; or if the red-field/buddie ratio is applicable in our aquarium. Instead, I would like to look at the big picture of how we account for the nutrients balance in our aquarium.

Chemical Engineering 101

When it comes to understanding how nutrient level changes in the enclosed reef aquarium, I find it sensible to apply a basic chemical engineering Concept.

Input – Output + Generation – Consumption = Accumulation

In the context of reef nutrients in aquarium water, for instance, nitrogen, we can think of it in the following manner:

Input: How nitrogen is added into the system. Generally, nitrogen elements are added into the aquarium through feeding. At the same time, cyanobacteria fix some nitrogen from atmosphere.
Output: Surprisingly, there are very few ways to directly export nitrogen from water. What little output there is generally comes from the action of activated carbon, water change, or polymer resins.
Generation: When fish and coral carry out their biological activities, they produce nitrogenous wastes, such as ammonia. When microbe bacteria break down organic compounds or dead organism, some nitrogenous compounds are produced.
Consumption: Lucky for us, most living things in our reef tank consume nitrogen in one way or another. Fishes consume the food we feed, taking up some nitrogen. Certain Bacteria, algae and zooxanthellae consume nitrate as they grow. These processes help remove excess nitrogen from water.

Case Study

I feed my aquarium a mixture of pellets and coral food, say, 4 grams of nitrogen per day. This is the input. The activated carbon and zeolite extracts 0.5 gram from the water per day. This is the output. Out of the 4 grams of nitrogen in the food, 2 grams are eaten by the fishes and coral. This is the consumption. The bacteria and algae also make use of the nitrogenous compound in the water. As the result, they increase their biomass by 1 gram of nitrogen. This is also consumption. Fishes release 0.5 gram of nitrogen in to water in the form of ammonia. This is generation.

As the result, for this aquarium, the accumulation of nitrogen is:

Accumulation = 4 – 0.5 + 0.5 – 2 – 1 = 1 gram

Diagram illustrates the processes which increase (blue) and decrease (red) nitrogen in water

This means the nitrogen content of the aquarium will increase by 1 gram per day. This also means, for a 500 L aquarium, the nitrate will increase by about 9 ppm per day.

Hidden Cost of Nutrient Export

Get ready for an “eureka” moment!

If we can export nutrients simply by having more bacteria growth, does that mean there is no limit to the amount of nutrients a system can handle? If it sound too good to be true, it probable is. Unfortunately, There is a hidden cost for an aquarium to export these nutrients.

Nitrobacter sp. One of the bacteria involved in nitrification. It consumes oxygen as it work.

When bacteria breaks down organic compounds, they consume oxygen and gives out carbon dioxide. When bacteria make use of C, N and P to grow and multiply, they consume oxygen and give out carbon dioxide. The hidden cost for nutrient export is dissolved oxygen. We can have gazillion bacteria to help us deal with all the nutrients. However, when they consume all the oxygen in the aquarium, things will start to die, fast.

Here we are, back to square one, the chemical balance of nutrients is ultimately limited by dissolved gases. This decides how many fishes and coral we can keep in an aquarium.

Mineral Balances

The balances of reef-building minerals are critical for corals especially so in an enclosed boxed aquarium tank. At the same time, they also have a measure of impact on fishes and invertebrates affecting their growth and health. In a reef environment, these minerals need to remain within an acceptable range. For a quick recap of these minerals, you can read this excellent article by Chun Wai.

When coral consume these minerals, we must add them back into water. Some of the techniques adds back the consumed ions in a balanced way, while others may upset the ratio of ions in reef water. We shall briefly discuss some popular methods.

Kalkwasser: this unbalanced method increases calcium and alkalinity, without topping up magnesium and other trace elements. it also increases pH slightly. The adding of kalkwasser depends on evaporation rate.
2- or 3-part dosing: this unbalanced method can increase major and trace elements. However, it has some impact on the salinity and ionic balance.
Balling: this balanced method will not affect the ionic balance or salinity of the reef.
Calcium Reactor: this balanced method will not affect the ionic balance or salinity. However, it tends to increase the dissolved carbon dioxide in water. This puts pressure on gas exchange.

Back to Bioload

We have covered quite a few topics in this article. Let’s return to the original question: So what is stopping us from adding more fishes?

Mixing of microbubbles and water in a protein skimmer creates plenty of surface area for gas exchange

My answer is : gas exchange. We need to maintain a reasonable level of oxygen, while removing carbon dioxide. We do this not only to ensure the fishes and coral will stay alive, but also in support of the nutrient cycles through bacteria. Therefore, I believe the rate of gas exchange is the limiting factor for the bioload in reef aquarium. Keeping this in mind, it makes sense to have a largest possible nutrient export method, for example, an oversized protein skimmer, or largest algae refugium; So that your aquarium has a higher limit for bioload.

Author - @JiaEn