This works for any size of tank. I use it for a 75 gallon tank (pH 6.6, KH 4). Here is a link to a German forum with some instructions on how to build it (you can use glass or plastic glas): http://forum.zierfischverzeichnis.de...light=Paffrath
The second version is for labyrinth fish and should be placed near the water surface because of the ventilation. However, the amount of CO2 diffusing into the water does not depend on the height of the bell (1 to 1.5 inches is good) or how deep you place it in the water. It is determined by the surface area that is in contact with the CO2 under the bell.
The size (area) of the bell depends on your KH and the size of your tank:
For each 100 liters (25 gallons) of water:
kh: < 10 -> 30cm² (4.8 sq in)
11 -> 50cm² (8 sq in)
12 -> 70cm²
13 -> 90cm²
14 -> 110cm²
15 -> 130cm²
16 -> 150cm²
This keeps the CO2 level constant at about 20-30 ppm (depends on water movement below the bell).