Tools

CO2 calculator

Estimate dissolved CO2 from KH and pH values. Use the chart to visualize target ranges and confirm your reading.

KH + pH to CO2 ppm

Freshwater focus

Calculate dissolved CO2

Use the standard KH/pH relationship for planted freshwater aquariums.

Saltwater systems need a different CO2 approach. Use marine-specific guidelines for reef tanks.

CO2 reference chart

Cross-reference KH and pH values to compare your result with typical CO2 ranges for planted tanks.

KH
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
1
30
24
19
15
12
9
8
6
5
4
3
2
2
2
1
1
1
1
0
0
0
2
60
48
38
30
24
19
15
12
10
8
6
5
4
3
2
2
2
1
1
1
1
3
90
71
57
45
36
28
23
18
14
11
9
7
6
5
4
3
2
2
1
1
1
4
120
95
76
60
48
38
30
24
19
15
12
10
8
6
5
4
3
2
2
2
1
5
150
119
95
75
60
47
38
30
24
19
15
12
9
8
6
5
4
3
2
2
2
6
180
143
114
90
72
57
45
36
29
23
18
14
11
9
7
6
5
4
3
2
2
7
210
167
133
105
84
66
53
42
33
26
21
17
13
11
8
7
5
4
3
3
2
8
240
191
151
120
96
76
60
48
38
30
24
19
15
12
10
8
6
5
4
3
2
9
270
214
170
135
107
85
68
54
43
34
27
21
17
14
11
9
7
5
4
3
3
10
300
238
189
150
119
95
75
60
48
38
30
24
19
15
12
9
8
6
5
4
3
11
330
262
208
165
131
104
83
66
52
42
33
26
21
17
13
10
8
7
5
4
3
12
360
286
227
180
143
114
90
72
57
45
36
29
23
18
14
11
9
7
6
5
4
13
390
310
246
195
155
123
98
78
62
49
39
31
25
20
16
12
10
8
6
5
4
14
420
334
265
210
167
133
105
84
67
53
42
33
27
21
17
13
11
8
7
5
4
15
450
357
284
226
179
142
113
90
71
57
45
36
28
23
18
14
11
9
7
6
5
16
480
381
303
241
191
152
121
96
76
60
48
38
30
24
19
15
12
10
8
6
5
17
510
405
322
256
203
161
128
102
81
64
51
41
32
26
20
16
13
10
8
6
5
18
540
429
341
271
215
171
136
108
86
68
54
43
34
27
21
17
14
11
9
7
5
19
570
453
360
286
227
180
143
114
90
72
57
45
36
29
23
18
14
11
9
7
6
20
600
477
379
301
239
190
151
120
95
76
60
48
38
30
24
19
15
12
10
8
6
KH (rows)pH (columns)

KH + pH estimate

Enter carbonate hardness and pH to estimate dissolved CO2 in ppm for freshwater planted aquariums.

Chart for quick comparison

Use the reference chart to compare nearby KH and pH values and understand how small pH shifts affect CO2.

Safety-first interpretation

High CO2 can stress fish and shrimp. Make changes gradually and monitor livestock closely.

Planted tank CO2 guide

Estimate CO2 from KH and pH, then verify with livestock behavior.

The KH/pH CO2 relationship is a useful planted tank reference, but it is still an estimate. Use it with observation, surface agitation, drop checker context, and careful changes to injection rate.

How the CO2 calculator works

The calculator uses the common planted aquarium formula CO2 ppm = 3 * KH * 10^(7 - pH). KH is entered in dKH and pH is the measured aquarium pH.

This relationship assumes carbonate buffering is the main factor affecting pH. Other acids, buffers, active substrates, and water chemistry can make the estimate less accurate.

Using CO2 estimates in planted tanks

Many high-light planted tanks aim for a stable CO2 range before and during the photoperiod, but livestock safety comes first.

If fish gasp, shrimp become inactive, or livestock gather near the surface, reduce CO2 and increase aeration even if the calculator suggests a target range.

Why pH timing matters

CO2 changes throughout the day. A pH reading before CO2 turns on, during peak injection, and after lights out can tell very different stories.

For useful comparisons, test at consistent times and log the readings together with bubble rate, drop checker color, and plant response.

When KH and pH are not enough

Active aquasoils, peat, driftwood, acids, remineralizers, and non-carbonate buffers can change pH independently of dissolved CO2.

Use the calculator as a reference, not a guarantee. Combine it with a calibrated pH test, KH test, livestock behavior, and gradual adjustments.

FAQ

CO2 Calculator FAQ

What CO2 level is good for a planted aquarium?

Many planted tanks aim somewhere around 20 to 30 ppm, but the right level depends on livestock, oxygenation, plant demand, light, and stability.

Is the KH/pH CO2 calculator accurate?

It is an estimate. It is most useful when carbonate hardness is the main buffer and pH is measured consistently with reliable tests.

Can too much CO2 hurt fish?

Yes. High CO2 can stress or kill fish and shrimp. Watch behavior closely and increase aeration or reduce injection if livestock show distress.

Why did my pH drop after adding CO2?

Dissolved CO2 forms carbonic acid, which lowers pH. The size of the pH drop depends on KH, injection rate, gas exchange, and timing.

Does this calculator work for saltwater aquariums?

No. This tool is intended for freshwater planted aquariums. Saltwater and reef systems need different interpretation.

When should I measure pH for a CO2 estimate?

Measure pH at a consistent point in the CO2 schedule, such as before injection and again near peak injection. Comparing readings at the same time each day makes trends easier to interpret.

Related aquarium planning

Continue with the next aquarium decision.

Use the calculator result with logs, guides, and maintenance tools so one number becomes part of a repeatable aquarium care routine.

Nitrate & Phosphate Ratio CalculatorCheck nutrient context when troubleshooting plant growth.Water Parameters TrackingTrack KH, pH, CO2 context, and repeated readings over time.Aquarium Test Results LogLearn why repeated measurements are more useful than one test.Aquarium CO2 Tracking GuideConnect KH, pH, injection timing, plant response, and livestock safety.

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