What is CO₂?

Carbon is a basic building block of all living beings, including plants, of course. Plants absorb carbon in the form of CO₂, i.e. carbon dioxide. CO₂ is therefore a decisive factor for healthy plant growth.

In addition to CO₂, light and micro- as well as macronutrients to the basic requirements of plant growth. If these "ingredients" are in the right proportion, the plant does Photosynthesis and builds the organic matter required for growth.

If you want to keep demanding plants with a lot of light, CO₂ fertilization is strongly recommended. With the CO₂ supply you ensure that your plants do not get symptoms of deficiency, grow healthily and prevent algae from spreading.

However, CO₂ addition is not always necessary. In aquariums with less demanding plants and low to medium-strong lighting, a CO₂ is not needed. If you decide to set up a such a low tech tank, look here for a detailed shopping list which get's you started.

If you're interested in how plants grow with and without CO₂, this video shows the development of equally planted Aquariums with and without the addition of CO₂.

Overview: The different CO₂ systems

There are two established types of CO₂ supply for your tank. With a Bio-CO₂ system the CO₂ is produced by fermenting yeast in a sugary solution. Another possibility is a pressurized gas system. Here the CO₂ is produced industrially and comes under high pressure in a gas cylinder. Another, not yet widely used option is the "Chemical CO₂ system". The CO₂ is produced on site through the reaction of soda and citric acid.

The difference between the various systems is explained below.

The pressurized gas CO₂ system

A pressurized gas system is usually the combination of a pressurized gas CO₂ bottle in sizes from 500 g, 2 kg up to 10 kg, a pressure reducer and a needle valve. The CO₂ in the bottle is under high pressure under which it is in liquid form. If you let the CO₂ escape from the bottle it can expand and transitions to the gaseous state.

This high pressure makes it impossible to connect the bottle directly to a hose and to connect the CO₂ to the aquarium via a diffuser or reactor (more on this below). A necessary element of the pressurized gas system is therefore a pressure reducer. It ensures that the high cylinder pressure of over 50 bar is brought to a so-called working pressure is regulated down by approx. 2 bar.

However, the pressure reducer itself is still too imprecise to generate the required amount of CO₂ that is to be continuously fed to the aquarium. Therefore a needle valve is attached after the pressure reducer. It's through with this one An adjusting screw is possible to precisely adjust the amount of CO₂ introduced.

Night shut-off valve

Since the plants only do photosynthesis when sufficient light is available, it makes sense to use CO₂ pressurized gas systems with a night shutdown valve. This is an electrically switchable valve, a so-called solenoid valve, which is usually attached directly after the pressure reducer. In the picture above it has the number 6.

The valve is only opened with a timer if the lighting is also switched on. Alternatively, the valve can be switched on 30 minutes to an hour before the lighting and 30 minutes to an hour before the lights are switched off. This ensures that CO₂ is present exactly when the plants need it.

If the valve switches is switched on for eight hours, the CO₂ bottle will last three times as long as without night shutdown.

Pressurized CO₂: Component summary

Below find a list of components which are needed or optional for a pressurized system.

Where to fill the empty CO₂ bottle?

If the CO₂ bottle is empty, it must be refilled. This can usually happened in three places: the gas trade, the beverage trade or the pet shop. The gas trade probably has the best offer in terms of price. However, it is not uncommon that your bottle will be exchanged for another, already filled bottle and the empty Bottle will remain with the dealer. It will be refilled there and returned to the cycle. The advantage is that you always get a well maintained bottle. But it may be that you get a bottle with different dimensions and thus there may be space problems in the base cabinet.

Another option is beverage retailers. Many beverage retailers have the option to fill your bottle. Here it is rather unusual that your bottle against a other is exchanged. So you usually go with your own, freshly filled one Bottle home.

You can also have your bottle filled at the pet shop. Here you always get your own bottle filled up. The filling will probably be the most expensive in the pet shop. As an example: A filling of my 2 kg CO₂ bottle at the local pet shop costs € 17.99.

Be sure to check for leaks after changing the bottle. You can do that with, for example special leak detection spray or simply with washing-up liquid. Mix a small amount of water with a little washing-up liquid. With your finger or a syringe you can now apply the mixture to the screw connections. If you see it foaming or bubbling somewhere you have a leak and should double check the screw connection and tighten it if necessary.

Use of Sodastream CO₂ bottles at the aquarium

Sodastream bottles can usually be used via commercially available adapters. The adapters can be found in online shops, for example on eBay or Amazon.

The use of sodastream CO₂ bottles offers the advantage that you You can exchange empty bottles for full ones in many supermarkets and drugstores. For most people, the way to the supermarket is probably much shorter than the one to the nearest pet shop or gas dealer. A clear gain in comfort.

The soda stream bottles only contain 425 g of CO₂ however. The costs Compared to filling a larger bottle at a gas or pet store are significantly higher.

Chemical CO₂ system

A “chemical CO₂ system” is based on the reaction of Soda and citric acid substances interact with each other.

The system consists of a stainless steel cylinder and a pressure reducer, a needle valve, possibly a solenoid valve and mostly a bubble counter. The chemicals can be filled in via unscrewing the pressure reducer.

The chemicals sodium, citric acid and water are mixed in the stainless steel cylinder usually in a ratio of 1: 1: 2. The reaction starts immediately and it becomes CO₂ released. Now the pressure reducer together with the needle valve and any Night shut-off screwed onto the cylinder.

The advantage over Bio-CO₂ is that this reaction runs continuously and is not temperature dependent. The reaction is also independent of the pressure in the container. This means that night shutdown is possible.

For a 30-40 liter tank 400 g each of baking soda and citric acid should last about 100 days according to this video from AquaOwner. If you buy baking soda and citric acid in quantities of one kilogram, the cost of one filling is around 8 euros. A real alternative to the pressurized gas system.

Introduce CO₂ into the aquarium

The aim of a CO₂ system is to dissolve the CO₂ in the aquarium water to make it available to the plants. To make this as efficient as possible, the area of the CO₂ should be as large as possible when it is led past the water. This can can be achieved, for example, by “atomizing” the CO₂ into the smallest possible bubbles.

The methods presented below all have their advantages and disadvantages in terms of optics or the effectiveness with which the CO₂ is dissolved in the water. Below you will find a table showing a brief overview of the various Methods. A detailed description of each can be found in the following sections.

Bubblers / Diffusers

Diffusers are the easiest and cheapest way to get CO₂ into the aquarium. The diffuser is usually a kind of bell made of glass, the opening of which is covered with a membrane This membrane is very fine-pored. When CO₂ with pressure behind the membrane is introduced, the CO₂ flows through the microscopic Openings in the membrane and countless fine bubbles arise in the aquarium water.

The resulting fine atomization of the CO₂ creates a large surface. In this way, the CO₂ should be dissolved quite well in the aquarium water.

Advantages are the simple technique. The diffuser consists of one to a few components. Accordingly, not much can break. In addition, diffusers are available as glassware and are therefore quite attractive.

A disadvantage is certainly that the diffuser has to be placed in the aquarium and possibly visible to the viewer. Not everyone likes this. In addition, inner or outer reactors offer higher efficiency.

Inline Diffusers

Inline diffusers are looped into the outlet hose of the external filter. The principle is the same as at the diffuser. A membrane ensures the fine atomization of the CO₂ and through flow of water whirling the bubbles around, the inline diffuser achieves a very high efficiency.

The advantage is that the inline diffuser is not installed in the aquarium and is therefore invisible. Due to the long contact of the CO₂ bubbles and the turbulence with the water, it also achieves a high level of efficiency.

The higher costs, the more complex construction of an in-line diffuser and the more complex installation can be seen as a disadvantage. The device usually has to be removed from the filter circuit and be opened for cleaning. Only then you can access to the membrane. Furthermore, a very fine CO₂ mist often spreads in the aquarium. What efficiency is beneficial, the appearance of the aquarium may be detrimental.

The correct CO₂ concentration

If the CO₂ concentration in the water is too low, it has virtually no effect. Too high a concentration is fatal to living beings. It is therefore important to aim for an average value of 20 to 30 parts per million (ppm).

Monitor the CO₂ concentration of the aquarium

So-called CO₂ checkers can be used to monitor the CO₂ concentration. These are mostly spherical or teardrop-shaped structures made of glass or plastic - so-called drop checkers. A liquid is introduced into it that assumes certain colors for certain CO₂ levels in the water. Blue usually signals too low CO₂ level, green to light green are signals an optimum level. Yellow indicates that the CO₂ concentration is too high.

The CO₂ Checker liquid is usually available in two versions. The one variant changes from blue to green at a CO₂ concentration of 20 ppm, the other variant at a concentration of 30 ppm. For heavily planted aquariums, a value of 30 ppm should be aimed for.

Be careful: The liquid reacts with a significant time delay to changes of the CO₂ concentration in the water. Usually in the range of over 60 minutes. So you cannot expect to increase the CO₂ inflow with the needle valve and immediately see a color change in the liquid. The inhabitants of the tank can "feel" the change before you can see it. So slowly adjust the inflow via the needle valve over several hours and monitor the liquids color in case your CO₂ levels are off.

The bubble counter helps with the adjustment

A bubble counter is used for simple visual control of the inflowing CO₂ amount when adjusting the needle valve. The number of bubbles quickly tells you how strong when you turn the needle valve you have changed the amount of CO₂ flowing in. This is how you get a better sense of whether you've just made a big or small change.

However, a bubble counter does not give you any information about the current CO₂ content of the water. The bubble counter is therefore not a substitute for continuous monitoring of the CO₂ content of the water. It is only to be understood as an adjustment aid. See the previous section.