So you have started to inject CO2 into your planted tank, but you are unsure whether you have too much/too little.
If you are injecting too little CO2, there may be the possibility that plants are starved for carbon. If the "triangle" is unbalanced, recall that algae can take over.
Conversely, if you are injecting too much carbon dioxide, it is possible to suffocate your fish/invertebrates. The goal is to find a happy medium between these two extremes (around 30 ppm of CO2 is desirable).
Measuring CO2 can be accomplished in several manners.
1) Electronic CO2 meter
A very expensive piece of equipment, not many aquarists have the budget necessary for one of these pieces of high tech gadgetry.
2) Measuring pH/kH
Measuring pH and kH is a method of determining the amount of CO2 in your aquarium. This method relies on the fact that there exists a relationship between pH, kH and the amount of CO2 that can be dissolved. However, one must keep in mind that this method is only an estimate; there may be other buffers in the water that may skew results from this method.
An excellent article by Chuck Gadd as well as an in-browser CO2 calculator and CO2 reference chart can be found here:
3) Drop checker
Most likely the newest addition to measuring CO2 levels, a drop checker consists of an airspace between the liquid inside the drop checker and the water in the aquarium. Carbon dioxide readily diffuses outwards from water into the air; as such, the carbon dioxide in the aquarium will readily diffuse into the airspace in the drop checker. The liquid inside the drop checker contains a solution of known kH (i.e. 4 or 5 dkH) with an indicator (bromothymol blue (BTB)) which serves as a good indicator of CO2 dissolution. The CO2 that is in the airspace of the drop checker will readily diffuse into the drop checker solution, changing the colour of the BTB indicator.
A guide to making your own DIY CO2 drop checker written by me (shameless plug here!) has been copied below for your reference.
After some reading on various forums, and since I started DIY CO2 in my 2.5 g nano, I decided to build a DIY CO2 drop checker on the whim (hey, it's reading week, I get a break from university midterms, I deserve to spend some time with my aquariums! :P)
Anyways, here's the equipment you will need, or rather, the stuff that I had lying around my house that I used:
1 Glass container with screw on lid
1 Silicon glue applicator/similar type glue applicator
1 Rubber O ring that is about the same size as the screw on lid
Power drill with appropriate drill bits
1 Suction cup
1 Zip tie
Glass container with lid
Glue applicator with the tip cut off so it will fit into the glass container
As mentioned, you'll have to cut the plastic applicator so that it fits within your container first. Next, you'll want to drill a hole in the cap.
Finally, you can put the O-ring onto the applicator, put the applicator in, and screw on the lid.
Next, I needed some way to mount the whole thing in my nano, so I used a spare suction cup and a plastic zip tie.
You'll notice the zip tie just goes through the suction cup (i.e. I just put a hole through the nipple of the suction cup).
After the mechanical work was done, I had to start the chemistry part (I love chemistry!)
Items you'll need to make a 4 dkH reference solution for the drop checker:
1 Graduated Cylinder
1 Scale that can measure to at least 0.1 gram accuracy
1 Box of Baking Soda
1 Bottle of Distilled water
Scale that I bought off a site, it was $11 (free shipping)
Baking soda (sodium bicarbonate), you can pick this up at your grocery store.
Distilled water, again, grocery store.
First, since a 4 dkH reference solution is needed, I made the necessary calculations and prepared a 40 dkH solution before diluting it 10 fold to 4 dkH.
To make a 40 dkH solution, you need 1.2 grams of baking soda per 1 L of water. I made up a 2 L batch, so I needed 2.4 grams (the bigger the batch, the more accurate your final solution will become).
Then, taking 50 mL of the 40 dkH solution (measured with a graduated cylinder), I poured it into a bottle and filled the last 450 mL with distilled water (giving me a 1:10 dilution = 4 dkH solution).
After you make your 4 dkH solution, measure out 5 mL, and using a pH test kit (i.e. I used the API pH test kit, any test kit that uses bromothymol blue will be fine for this purpose), put in at least double the recommended number of drops (i.e. API recommends 6, I put in 8-9 drops). This makes the resulting solution darker, and easier to read.
Here's the liquid inside the drop checker.
Everything put together, just awaiting me to strap the drop checker to the suction cup.
The whole contraption has been placed into my nano, around 4:48 pm. At the time of writing this up (5:35 pm) the top portion of the liquid is already starting to turn green.
6:28 pm now, it's definitely green