Updated: Mar 25
As was stated in part one, the primary inputs for a partially self-sustaining planted tank are light, nutrients, and water. Below, I'll examine each input and their components as they pertain to a planted freshwater aquarium.
The initial source of energy for nearly all ecosystems is light. Your planted tank is no exception. As light enters the tank, it “feeds” the autotrophic plants and algae, which build up additional compounds. These are distributed throughout the ecosystem by consumption, secretion, and decay.
Photosynthesis occurs within the visible light spectrum. Plants will absorb light from 400nm to 700nm and use the energy to grow vegetative material, flowers, or to germinate seeds. The ideal aquarium lighting for planted tanks will emit light along the full visible spectrum. Especially effective lights will include extra diodes in the 550 to 700 nm range, as this part of the visible spectrum is associated with optimum chlorophyll absorption. The more light chlorophyll absorbs, the more energy your plants have available for growth and flowering.
Photosynthetically Active Radiation
PAR, or photosynthetically active radiation, is the amount of light, regardless of wavelength, your plants are absorbing; think quantity, not quality.
Sometimes light intensity is measured in watts per gallon or lumens. These measurements are inaccurate for determining light intensity as it affects your plants. Watts per gallon is outdated, as it measures the electrical output of the fixture, not the light being emitted. Lumens is a biased measurement because it only accounts for the sensitivity of the human eye.
Therefore, Photosynthetically Active Radiation (PAR), measured as Photosynthetic Photon Flux (PPF, or micromoles) is the most accurate measurement of light intensity for the planted aquarium.
Ideal PAR levels are 75 - 100 PPF for low light plants, 100 - 200 PPF for medium light, and 200 PPF and above for high light plants. You can adjust the output of your light with a dimmer, or by adjusting the distance of the fixture from your tank to change the PAR rating within your tank. All planted aquarium LED fixture manufacturers will publish their lights’ PAR ratings. Don’t forget to convert from the labeled unit of measurement to PPF if necessary. Or convert the PPF numbers above to the labeled unit of measurement on your fixture.
As discussed in part one, nutrients are added to the partially self-sustaining planted tank via dry and wet fertilizers, fish and invertebrate food, and CO2. They are removed through plant trimmings, detritus removal, and occasional water changes.
Plant nutrients include, but are not limited to: carbon, nitrogen, phosphorus, potassium, iron, zinc, boron, copper, manganese, cobalt, molybdenum, vanadium, nickel, rubidium, and other trace elements.
Fish and invertebrate nutrients include, but are not limited to: proteins, fats, carbohydrates, calcium, phosphorus, iron, iodine, magnesium, sodium, chloride, potassium, sulfur, copper, zinc, and other minor minerals and trace elements.
During a water change or cleaning of the filter, excess nutrients in the form of nitrates and phosphates are removed. These excess nutrients are also removed in the form of plant tissues by trimming. However, we are also removing other useful nutrients which may not be overly abundant, and which may be crucial to plant growth, as well as fish and invertebrate health. Additionally, in a partially self-sustaining tank, a buildup of excess nitrates or phosphates shouldn't occur, as they are absorbed by plants. Consequently, nutrients in a partially self-sustaining planted aquarium leave the system mostly through plant trimmings and removal of filter detritus.
Plant trimming is necessary for beautification, and to prevent overgrowth. Overgrowth leads to lower levels of the aquarium becoming stagnant from impeded flow and lack of light. Low flow areas can build pockets of detritus, hydrogen sulfide gas, and cyanobacteria colonies.
Filter detritus should also be removed to prevent low flow. Clogged filter pads can prevent properly oxygenated water from reaching your biological filtration media.
Detritus in the substrate should be left alone, if you have the right type of substrate which won’t pack down and prevent oxygen from entering. (More on that in part three).
In addition to plant nutrients being removed from trimmings and filter cleaning, some are also simply used up and converted into other compounds. Carbon, for example, is added to the tank as CO2 gas, but is quickly converted to glucose, among other compounds. Carbon and other nutrients can be expended and need to be replenished long before they are removed in plant trimmings.
To reiterate, plant nutrients are added to the aquarium in the form of dry and wet fertilizers, and CO2 gas.
Dry fertilizers are added to the substrate when the tank is being set up (more on that in part 3). They provide an extended release of basic, yet crucial, nutrients to plant roots.
Wet fertilizers are added to the water column on a regular basis. Plants mostly absorb these nutrients through their leaves.
Some plants need dry fertilizers, as they absorb a majority of their nutrients via their root system, which is not effectively exposed to the water column. Other plants don’t have an extensive root system, or will grow roots into the water column, and can survive or thrive solely on wet fertilizers. Most aquarists, including myself, will use a combination of both. The key is to create a strong biological foundation for your plants. Healthy plants are the lynchpin in the self-sustaining machine.
Carbon is crucial for plant growth and respiration. Without carbon, plants will experience stunted growth, tissue necrosis, yellowing leaves, no new growth, and eventually death. Carbon is best added in gaseous form, as plants are adept at fixing CO2 gas and breaking it down into carbon. They can also absorb carbon in the form of a glutamate solution, but this is not nearly as effective.
The nutrients that are not removed as plant trimmings or filter cleanings are either (1) converted into other compounds within the growing plant tissues, or (2) they break down as decaying plant tissues are consumed by saprotrophs, or (3) they are excreted or secreted by the plant into the water column. Once in the water column, they either leave the tank via gas exchange (oxygen gas), or they break down within the water column (plant oils/alkaloids).
Fish and invertebrate nutrients need to be added to the aquarium, not because they have been removed, but because they were most likely never there to begin with. As stated in part one, there are not enough different species in a typical planted tank to sustain a full food web and all its trophic levels. Some algae eating fish may never need additional food added. So with enough algae growing, they would be considered self-sustaining. However, unless you have a population of shrimp regularly reproducing and providing nutrients for your carnivorous and omnivorous fish, your fish will need additional input in the form of prepared, live, or frozen fish foods.
The best fish foods also provide probiotics, vitamins, trace elements, and a balanced ratio of protein to fat to carbohydrates. Any excess food is broken down by bacteria and microfauna into its base elements, and then absorbed by your plants.
Water changes shouldn’t be necessary in a self-sustaining planted tank. The only time water changes should occur is if there is an unexpected buildup of a certain nutrient or toxic element, such as ammonia or copper. If it is a small build up, it can be removed with special filter media, but sometimes the concentration is high enough for an emergency water change to be necessary.
Additionally, water does regularly leave the system in the form of evaporation. As stated in part one, evaporation is the hydrogen and oxygen atoms from H2O leaving the tank. The dissolved solids will remain, and will increase in concentration as more pure water evaporates.
In the case of evaporation, only pure water leaves the aquarium, so only pure water should be added back in. Replacing evaporated water with reverse osmosis deionized (RODI) water ensures nothing extra is added to the tank which wasn’t taken out.
When tank water is removed through a water change, rather than evaporation, all the dissolved solids are removed with it. These dissolved solids include everything dissolved in the water column: from calcium, magnesium, nitrogen and phosphorus, to plant excretions, trace elements, and minor metals and minerals. Therefore, when adding water back into the tank, all the beneficial dissolved solids should be added back as well. This can be accomplished by matching the chemistry of the new water with the chemistry of the tank water. Pay particular attention to general hardness, carbonate hardness, pH, and trace elements.
The key to self-sustainability is to follow the inputs on their different paths through the tank. Does the energy from the light and the carbon from the CO2 gas ever leave the tank? Or do they cycle through different forms within the same four glass walls over and over again, never to be removed?
Now that we have a solid idea of what self-sustainability is, and the components which contribute to it, we’ll discuss in part three how to set up your self-sustaining planted tank. I'll let you know what equipment you’ll need, and why you’ll need it.
In part four, I will detail how to maintain a partially self-sustaining planted freshwater aquarium.
Boodleshire LLC 2022