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.
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.
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.