Carbon Dioxide Part Two: Estimating Concentration

by John Tullock

In Part One last week, I discussed the importance of carbon dioxide (CO2) in the planted aquarium, and described several types of equipment used for adding CO2 to improve plant growth. In this post, I will discuss how CO2 concentration can be estimated when the pH and alkalinity of the aquarium are known.

No method exists that would permit the aquarist to directly measure the carbon dioxide concentration of his aquarium. However, the concentration can be estimated closely enough for our purposes, since we want to maintain about 12 to 20 ppm of CO2 during daylight hours.

Below is a table presenting the carbon dioxide concentration in parts per million as a function of pH and alkalinity. The pH values are listed across the top of the table, with alkalinity values (three equivalent values depending upon units used) down the left side. Thus, for example, at pH = 7.0, the optimum CO2 concentration lies at an alkalinity between 4 and 4.5 dKH. From the table, it is easy to see that lower pH and alkalinity values allow water to dissolve much more CO2.

Most aquarists are already aware that the widest variety of plants grows in soft, slightly acid water. If you plan to use tap water in your planted tank, test it first. Water of high alkalinity, “hard” water, will make your life harder when you use it for aquatic plants. Instead, use distilled water mixed with your tap water to an alkalinity no higher than 5 dKH. If for some reason you wish to increase alkalinity, numerous products are available for this purpose. Test kits for both pH and alkalinity are widely available, inexpensive, and easy to use. (Always check any test kit against a known standard solution before using it for aquarium purposes.) Test your aquarium at least once a week, preferably at the same time of day, to make sure your plants are getting enough vital CO2.

NEXT: Installing and Testing Carbon Dioxide Systems

TABLE 1: CARBON DIOXIDE CONCENTRATION AS A FUNCTION OF pH AND ALKALINITY

pH-->	6.4	6.5	6.6	6.7	6.8	6.9	7	7.1	7.2	7.3	7.4
dKH	mg/L CaCO3	meq/L
3	54	1.07	43		27		16		11		7	4
3.5	63	1.25	50		31		19		13		8	5
4	71	1.43	57		36		21		14		9	6
4.5	80	1.61	64		40		24		16		10	6
5	89	1.79	71		45		27		18		11	7
5.5	98	1.96	79		49		29		20		12	8
6	107	2.14	86		54		32		21		13	9
6.5	116	2.32	93		58		35		23		15	9
7	125	2.50	100		63		38		25		16	10
7.5	134	2.68	107		67		40		27		17	11
8	143	2.86	114		71		43		29		18	11
8.5	152	3.04	121		76		46		30		19	12
9	161	3.21	129		80		48		32		20	13
9.5	170	3.39	136		85		51		34		21	14
10	179	3.57	143		89		54		36		22	14
10.5	188	3.75	150		94		56		38		23	15
11	196	3.93	157		98		59		39		25	16
11.5	205	4.11	164		103		62		41		26	16
12	214	4.29	171		107		64		43		27	17
12.5	223	4.46	179		112		67		45		28	18
13	232	4.64	186		116		70		46		29	19