Sustainability, in this case, refers to a closed system capable of maintaining itself without any external input. Creating a truly self-sustainable ecosystem is nearly impossible, as some components must be supplied externally. The external inputs in an aquarium are light, nutrients, and water.
We can get close to true sustainability with a partially self-sustainable ecosystem. This means, some inputs (light, nutrients, water) are required to offset the necessary outputs (plant tissues, evaporated water, detritus).
What is a Partially Self-Sustainable Ecosystem?
The first aspect to consider with a self-sustainable ecosystem is whether the system is open or closed. An open system has input and output from the external environment. A normal aquarium with frequent water changes, chemical supplementation, and daily feedings is an open system.
A closed system has no input or output from the external environment. A closed glass jar with living plants and animals inside which is never opened is a closed system.
Trying to create a truly closed planted aquarium ecosystem is impractical, especially with fish, and the need to occasionally trim and remove growing plant material.
Our goal is to bring the aquarium as close to a closed system as possible, by minimizing the inputs and outputs, without sacrificing what is necessary to maintain a healthy and beautiful system.
Inputs and Outputs
What are the inputs and outputs we are trying to minimize? It goes beyond just trying to make our workload lighter by reducing the need for weekly water changes. Self-sustainability is also about the accomplishment of creating a functional ecosystem which, when done correctly, balances itself to the degree we allow.
Light
The primary input for nearly all ecosystems is light. The energy from light changes forms dozens of times as it makes its way through the trophic levels.
Trophic levels are the steps energy takes through an ecosystem. The first levels are organisms which create their own food (autotrophs), like plants. The middle levels are heterotrophs like herbivores, omnivores, and carnivores that consume autotrophs and other heterotrophs. The last levels are saprotrophs (fungi, and bacteria), that consume decaying organic matter.
Light starts with photosynthetic organisms consumed by herbivores, which in turn are consumed by omnivores and carnivores, all the way up to their decaying tissues being consumed by saprotrophs like fungus and bacteria. The leftover nutrients from decaying tissues, once completely broken down to their base elements by bacteria, are then reabsorbed by plants, completing the cycle and light’s journey through the ecosystem.
The exact same process happens in our planted tanks, except we occasionally remove some of light’s energy in the form of plant trimmings. This output breaks the cycle and those tissues are not allowed to decay in the aquarium, depriving it of nutrients down the line. Which is why we must add nutrients back into the tank later.
Nutrients
Autotrophs get their nutrients by absorbing them directly from the water column or the substrate. These nutrients take the form of carbon, nitrogen, phosphorus, potassium, iron, zinc, boron, copper, manganese, cobalt, molybdenum, vanadium, nickel, rubidium, and other trace elements.
Some heterotrophs (fish, snails, and shrimp) are herbivores and get their nutrients from autotrophs (plants and algae). Others are omnivores or carnivores and require live or processed food. Either way, these nutrients come in the form of proteins, fats, carbohydrates, minor minerals, and trace elements.
Biodiversity is closely tied to nutrient intake. In a natural ecosystem, trophic levels are supported by thousands, and sometimes millions of species. In our aquarium, we may have a few hundred microscopic species, but only a few dozen macroscopic species at most. In the wild, fish eat insects, shrimp, plants, algae, eggs, etc, depending on their trophic level and niche. There are not enough species in most aquariums to support a developed food web, so fish and invertebrate foods need to be added.
We input nutrients into the planted aquarium through dry fertilizers, liquid fertilizers, CO2 gas, algae and plants, as well as live, dry, or frozen foods.
Water
As water from the tank evaporates, only the hydrogen and oxygen molecules disappear. All of the solid, dissolved components like salts, minerals, and organic compounds remain. As the pure water evaporates and the rest remains, the dissolved components increase in concentration. If the tank evaporates half-way, it would double the concentration of all the dissolved minerals, salts, and organics.
For this reason, only pure water should be added to the tank when replacing evaporated water. In nature, pure water is added to the ecosystem in the form of precipitation. While some rain does absorb nitrogen from the atmosphere, new water is considered to be functionally pure.
Pure water means water that is composed of just two hydrogen atoms and one oxygen atom, with no dissolved solids. To supply pure water to your aquarium, you should use reverse osmosis deionized (RODI) water. Distilled water loses some of its chemicals like chlorine, but still retains many dissolved solids and is not considered pure. Additionally, most “filtered” water is run through a simple floss with some carbon. This process removes large particles and some organic material, but is also not considered pure. This article on RODI water helps explain why this filtration process gets water as close to pure as possible.
In summation, light, fish food, major fertilizers (nitrogen, phosphorus, potassium), minor fertilizers (iron, trace elements) CO2, and water are the main inputs for a self-sustaining planted tank.
Outputs
The main outputs in a partially self-sustaining planted aquarium should be oxygen gas, plant tissues from trimmings, water, and a portion of the detritus. Oxygen gas is produced by plants and released into the water column. Because the aquarium is not sealed, gasses are allowed to escape, as is evaporated water.
Plants grow from the inputs discussed earlier and form them into tissues, glucose, and tertiary compounds like pigments, alkaloids, and oils. You can get even closer to self-sustainability by never trimming or removing plant tissues, allowing them to decompose in the tank and keeping the energy within. This “overgrown” look is a practiced aesthetic in the hobby, however, most aquarists choose to trim their plants for beautification. By doing so, we add an output to the self-sustainability equation.
Detritus that falls to the substrate should never be vacuumed or removed in a self-sustaining aquarium. Detritus left in the tank should be allowed to decompose and release nitrogen, phosphorus, carbon and other nutrients into the water column to be reabsorbed by plants. The proper substrate to use for this is discussed in part three.
However, the detritus collected by the filter should be removed once every few months, or the flow from the filter return will slow to a functionally useless rate. Ideally, the filter would only contain biological filtration, but a floss pad is usually necessary to keep the biological media from becoming clogged.
Self-Sustainability and Automation
In its truest sense, automating your processes with dosing pumps and controllable lights does not contribute to the definition of self-sustainability, but they can go hand in hand. If the goal is to have a tank which “takes care of itself”, then automating processes, and removing manual labor as much as possible is the greatest ally of self-sustainability.
Partial Self-Sustainability
A truly self-sustaining planted aquarium would be one in which we do not add any inputs ourselves. The light would stimulate plant growth, which would feed all the trophic levels. The nutrient and energy cycles would play out over and over again going from elements, to compounds, to molecules, and back again, all without any additional input. The plants would then grow to fill the aquarium and the bottom portions would die off, contributing to the nutrient and energy cycles once again.
However, usually, we want to trim those plants and aquascape a beautiful scene. Because plant trimmings are removed and water is evaporated, those outputs have to be replaced with additional inputs. We also have fish in the aquarium which require regular feedings. With these factors in mind, the ecosystem we have created can be considered partially self-sustained.
Part Two
In the next part of this series on creating a self-sustainable aquarium, I’ll go into detail on each of the input and output components. I’ll discuss light, nutrients, and water, and their contributing factors as they pertain to inputs and outputs.
In part three, I’ll cover the physical and mechanical components of these inputs and outputs, and how to build your self-sustaining aquarium from scratch.
In part four, I’ll discuss how to properly maintain your self-sustaining aquarium.