I have some plants in my tropical aquarium. They're not doing too well. I suspect I may have a CO2 shortage. Can I add soda water to the tank? I've seen complicated setups for adding CO2 to water. Soda water is cheap and pouring some in the tank every few days would be really simple. Would it work? Would it be safe for the fish?
Soda water uses carbonic acid to produce CO2 so in theory it should work, but it is not that easy: carbonic acid is as the name says an acid, and any type of acid will lower the water's pH too fast for the fish and to some degree too fast for the plants to adapt to the changed pH.
Carbonic acid dissolves calcium creating calcium carbonate, so if you have snails or any type of mussels in your water, their shells will start to dissolve.
Carbonic acid breaks down into water and CO2 but this process is dependent on how fast the CO2 is used/escaping from the water.
Read more about carbonic acid here https://en.wikipedia.org/wiki/Carbonic_acid.
So it is better to use CO2 in a form other than carbonic acid to help in plant growth, you will not avoid some carbonic acid beeing created but it will happen over a longer period and make the pH drop slower.
The little carbonic acid created will stabilize the pH and not make it crash like adding soda water will do.
If you still want to use soda water you will have to create a drip system so you can control the flow very exactly, you will also have to invest in equipment to measure the CO2 content in the water.
First we need to clarify, what is exactly in the bottled water:
To make it sparkle, the companies add CO2 gas. (In some dwells this gas was taken by the water, when dropping through the different layers of soil/stone). As long as the water is under pressure, this gas is dissolved in the water. Only around 0.2% of this dissolved CO2 will react with the water molecules (H2O) to build carbonic acid (H2CO3). The higher the pressure and the colder the water, the higher the ability of CO2 to dissolve (but not to react!) will be.
Once the bottle is opened, the pressure decreases; some CO2 will separate from the water and one can hear a hissing sound when it comes out. If the bottle stays open, more and more CO2 gas can separate from the water and little bubbles arise.
Now we know, there is a very low acid amount in bottled water. Around 99% of the CO2 is pure CO2 gas, the same like in professional CO2-tank-fertilizer-gas.
For the pH values of bottled water: The pH of "neutral" water is around 7, the one of bottled water could be low as 4.
What will happen, if one gives bottled water into a tank?
One needs to know, that in chemical reactions there is always some kind of balance between the starting product(s) and the resulting product(s). In the most less cases a chemical reaction will (without additional treatment) consume all starting product(s) and left only the resulting product(s). (You can imagine this like a kind of system of water pipes. If you open one tap and take some water, the whole system will change a little bit, until the water is in balance again.)
If one has lime in the tank, there will not only be limestone in it (CaCO3) but also some portion of dissolved Ca2+ and HCO3- ions because in aquatic environment there are equilibrium reactions:
CO2(aq) + H2O ⇋ H2CO3 ⇋ H+ + HCO3- ⇋ 2H+ + CO32-
CaCO3 + H+ ⇋ Ca2+ + CO32- + H+ ⇋ Ca2+ + HCO3-
Hydrogen ions (H+) which are the source of acidity are marked bold for visual clarity, to signify how these two reactions are entangled with each other.
So in the tank (yet without bottled water) we have a beautiful amount of chemicals...
Now we add CO2 and so we change the balance: more CO2(aq) is available! So the reactions consuming CO2 will be happy and happen over time to reach the balance again. The CO2 will react with water to form carbonic acid, which will then partially dissociate into H+ and HCO3- and because now the H+ concentration is increasing, the reaction of lime (CaCO3) dissolution will shift to produce more HCO3- while simultaneously consuming H+. As a result, the amount of undissolved limestone in the tank will decrease, and the surplus of H+ will be consumed in this process, preventing pH from decreasing.
Now, to connect the pH value with this:
As long as there is lime in the water, that the 0.2% of carbonic acid can react with, there is no problem in your tank, because the balance in this reaction is clearly at the side of lime. But if (if it is even possible) one has NO lime in the water, the amount of carbonic acid could increase when adding a lot of bottled water. It would be a spike, but it would not be persistent because to get the balance again, the carbonic acid would decompose into water and carbon dioxide, to get in the 0.2% to 99.8% balance again.
Adding bottled water in general should not be a risk for the life in the tank. It does not differ much from professional CO2 gas adding processes.
If one adds a high amount (maybe 1/3 to 1/2 of the original water volume) of bottled water, there could be spikes/peaks in some chemicals, that may be difficult to overcome for the life in the tank.
One should imitate the smooth and continuous supplementation of CO2 from the professional equipment to stay safe.
It is possible to calculate the somewhat precise amount of carbonated water that is safe to add; first, most aquarist sources about CO2 injection systems say that we should aim for about 35 ppm (mg/L) of dissolved CO2. Ambient CO2 concentration in aquarium without any CO2 equipment is non-zero and variable, around 0.6 to 3 ppm, but for simplicity in this example case we could assume 0 ppm to be "good" approximation.
Carbonated water sometimes states the precise concentration of dissolved CO2 (for example, let's consider one specific water brand which states 2200 mg/L). For instance, if an aquarium volume is 30 L, then adding the whole 1.5 L bottle of this water would bring 3300 mg (1.5 L * 2200 mg/L) of CO2 and increase the total water volume to 31.5 L; then we see that it would result in CO2 concentration of 104.8 mg/L (3300 mg / 31.5 L) which would be about 3 times more than safe value of 35 mg/L and thus dangerous; but if we add just 0.4 L of this water then it would result in about 29 mg/L which is within the safety zone.
One needs to have in mind, that the plants will use CO2 too to build up sugar and other typical biomolecules (and grow, which is the primary and ultimate reason to add CO2), so there will be leftover lime which can again bind with carbonic acid, without the need to supply additional lime every time you give bottled water into the tank. But the plants do it in a continuous and regular manner, and the bottled water should be given then in regular little amounts, too.
Also, one needs to have a close look to the other additional minerals the bottled water can contain. This could enrich in the water. But these minerals are different in each brand, and their effect to tanks is well known, so I do not re-write it here.
Von den Molekülen des gelösten Kohlenstoffdioxids reagieren allerdings nur etwa 0,2% mit den Wassermolekülen zur Kohlensäure. 99,8% der Kohlenstoffdioxidmoleküle sind einfach nur im Wasser gelöst.
 from German to English:
Only about 0.2% of the molecules of dissolved carbon dioxide react with water molecules to form carbonic acid. 99.8% of the carbon dioxide molecules are simply dissolved in the water.
Note that dissolved carbon dioxide in extracellular fluid is often called as "carbonic acid" in biochemistry literature, for historical reasons.
The hydration equilibrium constant at 25 °C is called KH, which in the case of carbonic acid is [H2CO3]/[CO2] ≈ 1.7×10−3 in pure water and ≈ 1.2×10−3 in seawater. Hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO2 molecules.
Die Fällung mit Kohlendioxid ist der am häufigsten eingesetzte Prozess, insbesondere in den On-site-Anlagen der Papierindustrie. Sauberer Kalkstein beziehungsweise Branntkalk wird zunächst zum Calciumhydroxid (Kalkmilch) gelöscht und anschließend als dünne Suspension dem Reaktionsbehälter zugeführt. Dort leitet man so lange Kohlendioxid ein, bis das Calciumhydroxid vollständig zu Calciumcarbonat umgesetzt ist. Die Reaktionsdauer kann durch den Verlauf des pH-Wertes beurteilt und gesteuert werden.
CaCO3 ⟶ CaO + CO2
CaO + H2O ⟶ Ca(OH)2
Ca(OH)2 + CO2 ⟶ CaCO3 + H2O
Während Leitungswasser und stilles Wasser einen neutralen pH-Wert um 7 haben, können bei Sprudelwasser pH-Werte um 5,5 gemessen werden.
 from German to English:
While tap water and non-carbonated water have a neutral pH value of around 7, sparkling water has a pH value of around 5.5.
Die Konzentrationen der drei (eigentlich vier) Kohlensäure-Spezies, also der freien Kohlensäure (H2CO3 und gelöstes CO2), des Hydrogencarbonats und des Carbonats sowie der Oxoniumionen stehen miteinander durch das Massenwirkungsgesetz in einem berechenbaren Zusammenhang. Die Konzentration der Oxoniumionen wird durch den pH-Wert ausgedrückt. Bei einem gegebenen pH-Wert ist somit das Mengenverhältnis der Spezies festgelegt.
 from German to English:
The concentrations of the three (actually four) carbonic acid species: free carbonic acid (H2CO3 and dissolved CO2), bicarbonate, carbonate, and oxonium ions are related to one another through the law of mass action. The concentration of oxonium ions is expressed in terms of pH. At a given pH, the quantitative ratio of the species is thus fixed.
Note: oxonium ions (Oxoniumionen) in this case is a more general term for hydronium ions; hydronium ions have chemical formula of H3O+ and in this context are just an alternative representation of hydrogen ions H+.
H3O+ is just the H+ ion attached to the water molecule; it is sometimes notated like this because H+, being just a bare proton, is extremely small and thus has extremely concentrated charge; for this reason it cannot freely exist in water solution and is always immediately attached to at least one water molecule.
Takashi Amano is arguably the most influential person of modern-day planted aquarium keeping. It is said that he put carbonated water in his tank and noticed flourishing growth of his aquatic plants.
Having said that you are probably fine with pouring soda water in your tank! Just without flavoring of course.
Although, it may seem inexpensive at first, you will probably go through cans of the stuff in no time and spend more money over time.
You should consider a setup such as this pressurized $100 setup from this guide:
I'm almost positive that in the first year it will be much cheaper than soda water of the same duration.
You may think even little amount of carbonated water may be fine, but let you be warned here - carbonated water is more acidic than you realize, it's pH is actually between 3 and 4! So it will drastically lower the pH of your tank, giving your fish immediate gill shock which will quickly kill them. I learned the hard way, let that be a strong warning.