well... there are 2 basic ways to measure CO2
(both based on the same principle). When you inject CO2
into water, it lowers the pH (don't worry - it isn't the same as moving the pH with chemicals or buffers... the fish don't notice this kind of pH swing). So, if we know the amount of buffers in your water (using a degrees Carbonic Hardness measurement (abbreviated KH
), and then we know the pH of your tank, we can use a chart to extrapolate how much CO2
is in the tank. Ideally, we want somewhere between 20-40 ppm CO2
in our tank. I always shoot for 30ppm. Below is what the chart looks like:
So, based on the chart, lets say you have a pH of 6.8, and a dKH
(degrees carbonic hardness) of 7. That puts your co2
right in the sweet spot of 34ppm. Hopefully you already have a pH test kit... Any pet or aquarium store will sell a KH
test kit for 5 dollars or so.
Now, for the ingenious, often preferred way to test your CO2
: An aquarium drop checker. It works on the same principal I described above. It measures pH, and indicates a certain color based on how much CO2
you have in the tank. You fill it with water of a known carbonic hardness (KH
), and a few drops of pH indicating solution. The best value is 4 degrees KH
water (you can make this using baking soda and distilled water). With the drop checker filled with 4dKH water and some pH indicating solution, it will always let you know at a glance where your CO2
levels are at. If the solution is blue, your pH is high, and it tells you the CO2
is less than 20 or so PPM
. If it is dark green, it is around ~25ppm. If it is lighter green, it will be around ~30ppm co2
. If it is yellow, than you are above 35 or so ppm
. The drop checker pictured below is reading a nice green color, so the CO2
in this tank is just about perfect. Here is what a drop checker looks like:
Now, mixing up the 4dKH solution (baking soda and water) can be tricky, as it requires a pretty sensitive scale and some very small measurements. The drop checker will function less accurately, but still give you an idea, if you just fill it with some tank water (the directions that come with the checkers tell you to do that anyway... but the consensus based on scientific fact show that the most accurate way to use it is to use a specific water/baking soda solution that is measured at 4dKH).
The drop checkers you can get on ebay (search for "CO2
drop checker) for the best price. They range in price from a rediculous $40 for an official ADA drop checker, to a chinese knock off that looks almost the same (and definitely functions EXACTLY the same) for $5-6. Some folks are handy enough to actually build their own for next to nothing, with things laying around the house.
The drop checker is great because it will always let you know how you are doing with CO2
, but it does lag just a bit. It will be about 15-30 minutes behind what is actually happening in the tank, but that is close enough to give you an accurate picture of what is going on.
Now, as for DIY, you do not shut it off, it is a continuous process. This means when the lights go out and your plants are not using CO2
levels will rise in the tank. You definitely have to watch this. If it is getting too high for comfort, you can set an airstone on an airpump opposite the timer for the lights. This way, the bubbles will help get rid of the co2
in the water while the plants are not using it. During the day, the air needs to be shut off so that the co2
stays in the water and your plants use it. It is all going to be based on a little trial and error, and some testing, tinkering, and adjusting. It sounds intimidating at first, but you will get the hang of it quick, and once you figure out what is working for your tank, it won't require much adjustment.
Anyway, sorry I wrote you a book. Let us know if you have more if this spurs more questions!