Every planted tank keeper has heard of proper lighting and adding carbon dioxide (CO2) to an aquarium. Products to help aquarists achieve both of these goals can be found in most local fish stores, online and in most aquarium magazines.
Both lighting and CO2 are often misunderstood, and misinformation persists in the hobby. Lighting and CO2 are so tied together that it is best to discuss both aspects in the same space. The two inputs are tied together because of the photosynthetic process, and it is impossible to understand this process without understanding both light and CO2.
Complicated information about CO2 is ubiquitous in the hobby, and many aquarists decide to forgo using CO2 in their aquariums because it seems so difficult. I can tell you that, while it does take some study and experimentation, providing adequate lighting and injecting CO2 into a planted aquarium aren’t as hard as they’re made out to be.
CO2 Basics
Let’s start by covering some basics of CO2. Plants use carbon dioxide – that’s one carbon atom and two oxygen atoms – in the process of photosynthesis. In combination with energy derived from light photons and certain enzymes the plants produce, CO2 is converted into carbohydrates, in a process called carbon fixation. The plants use carbohydrates produced through this process as food, essentially.
Of course, the actual chemistry of this process is very complicated and I won’t go into the specifics here. The best way for the average planted tank enthusiast to think of CO2 is as a nutrient for plants.
When growing plants, we have to provide them with various nutrients to help them grow. Sunlight is the most important input, providing energy for all of the necessary chemical reactions. We also have to include macro- and micronutrients, such as nitrogen, among many others. One of the most important and often overlooked nutrients we add is CO2.
Limiting Factors
Because it is so often overlooked and poorly understood, the absence of CO2 is often the limiting factor in plant growth. The most important factor is the availability of light of the correct spectrum, but it is generally easier to provide the right amount and type of light, thanks to all of the quality lamps we see in the hobby.
Therefore, if you are having trouble getting your plants to grow and thrive in the way you prefer, you should first check to make sure you have the correct amount and quality of light. After checking your lighting, the most likely factor limiting plant growth is the availability of CO2 in your aquarium.
Problems with Measurement
Now, adding the correct amounts of both light and CO2 is relatively easy to do in a general sense, but when it comes to measuring specifics, we run into trouble. For example, a common method of quantifying the amount of light plants require is to relate the amount of light made available to the quantity of water in the system. This is often referred to as the “watts-per-gallon rule.” A common form of this rule says we should provide 3 watts of light for every gallon in an aquarium.
In my experience, this rule is a very general guideline that often has little applicability to the average aquarist. There are just too many factors affecting light availability in any given system for the watts-per-gallon rule to translate in any meaningful way between various setups.
For example, light intensity from a single point source is inversely proportional to the square of the distance from that source. This is referred to as the “inverse square law.” It means that any object that is twice as far away from the light source as another object of equal size will receive only a quarter of the energy that the closer object receives.
Therefore, taller aquariums will experience a greater loss of light energy near the bottom compared to shallower aquariums with the same volume of water. Of course, this makes relating the amount of light necessary for plants in each aquarium problematic if we are using the watts-per-gallon rule. Providing the same number of watts for the taller aquarium may not be successful because of the inverse square law.
This examination is an oversimplification of how the inverse square law affects light intensity in the aquarium, and many other factors must be taken into account when considering how light behaves in an aquarium. However, this should give you an idea of why the watts-per-gallon rule is inadequate.
Incompatibility of Universal Rules
This is only one example of why the watts-per-gallon rule fails in aquaria. There are scads of other factors reducing the effectiveness of using this rule. Total plant biomass, plant growth rates, aquascape design, water quality, the presence of reflectors and the types of plants used all change a system’s lighting requirements. Therefore, the watts-per-gallon rule is only a general guide at best, and at worst it represents a total misunderstanding of a system’s light requirements.
The same problems of measurement affect the addition of CO2 to the aquarium. Plant growth rates, availability of other nutrients, the presence of adequate light, gas exchange rates at the water surface and the chemistry of the water itself all affect the levels of dissolved CO2 in an aquarium.
Newer planted setups generally benefit from the addition of CO2 more than older, more established planted setups. Certain plants are better equipped to use CO2 in aquaria. It’s even possible for specific areas within an aquarium to see higher levels of CO2 because of inadequate water flow throughout the system.
Building a Better Planted Tank
I’m bringing all of these problems up not to refute my argument that adding the correct amounts of CO2 and light is easier than it’s made out to be. Rather, I’m trying to highlight the problems aquarists face when trying to make sense of the whole issue. In researching these issues, many aquarists run into problems either understanding the application of these ideas or in their actual implementation.
Both problems can be remedied, however. In the future, we’ll work to simplify these issues and get at some guidelines that will aid in understanding these issues and provide for their correct application. When we dispel some of the myths about lighting and CO2, I think we’ll all be able to master their application in our aquariums much more easily.
Ultimately, I hope to create a practical, general guide that will allow aquarists to understand the processes enough to measure their own parameters and provide the correct amount of light and CO2 for their setups. I’m going to cover other aspects of these processes, too, but at the end of the discussion we should have a clear set of procedures for setting up a reasonably well-balanced, easy-to-understand aquarium system that incorporates correct lighting and CO2 injection.
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