Our Pressurized CO2 System

Why CO2? :After you have provided good lighting, substrate and nutrients for your plants, they are all set to flourish but for one essential nutrient -- carbon. In a tank with good aeration, the CO2 will only be about 2-3 ppm. This isn't enough for rapid growth. By reducing surface agitation to a minimum and supplying CO2 to the tank, we create an environment in which the plants are no longer carbon-limited. (Oxygen for the fish is supplied by the plants rather than by the air.)). .

Setup: A 5-lb pressurized CO2 cylinder (C) is secured in an upright position by wall-mounted straps (S) and attached to a one-stage CO2 regulator (Reg) with gauges measuring cylinder pressure (cp) and exit pressure (ep). Fine control is afforded by a needle valve (NV). The CO2 flows through silicon tubing (T) through a glass tube to the bottom of the bubble counter (B) partially filled with water. Check valves (CV) prevent the flow of water back toward the regulator in case the cylinder should empty.

From the bubble counter a short piece of tubing connects to the top of the 12 x 2 inch cylindrical reactor (R). Water, circulated by a water pump (WP) at approx. 60-90 gph, enters from the tank into the top of the reactor, where it mixes with the CO2, and out the bottom, back to the tank. A ball valve (not shown) regulates the water flow to the pump. The reactor is filled with plastic balls that help break up the water stream and increase the surface area of contact between the water and CO2.

Tank Pressures: All valves are first closed. Main cylinder valve (V) is opened completely. The regulator knob (K) is opened just enough to give a low reading on the exit-pressure gauge. Cylinder pressure will read about 950-100 psi as long as there is some liquid in the cylinder. After a few months, when the cylinder is almost empty (i.e. there is only gaseous and no liquid CO2 in the cylinder) the cylinder pressure will gradually fall, at which time it's a good idea to refill, since the output pressure can actually start to rise at this point. We maintain an exit pressure of 5 LPM (liters per minute) or about 10 CFH (cubic feet per hour).

Adjusting the CO2 level : The bubble rate, which determines the rate that CO2 is delivered to the tank, is adjusted using the needle valve. However you won't immediately know the amount of CO2 dissolved in the water: bubble sizes, reactor or diffusor type, tank size and uptake (number, type, growth rate of plants) all vary. So you need to adjust your bubble rate until you get your desired CO2 level. You can usually safely start at about 30 bubbles/min and increase daily as required. You must use your pH and KH readings and the Krib Chart & Tables to monitor your CO2 level (CO2 test kits are also available). Below about 5 ppm, CO2 won't be of much help to the plants; much above 35 ppm can be toxic to your fish. 10-25 ppm is a good target range. Make sure your KH is at least 2-3 before you put any CO2 into your tank -- otherwise you can have a drastic drop in pH. Use sodium bicarbonate (baking soda) to raise your KH if necessary. One teaspoon will raise 50 gal water by 20 ppm (about 1.1 dH).

There is a strict interrelationship between pH, KH and CO2. Within limits, you can adjust your KH and bubble rate to regulate your pH and CO2 concentration. (Note that while the chart is generally accurate, the use of peat filtration or some commercial pH buffers can render it inaccurate.)

Raising your KH will raise your pH (no change in CO2).
Lowering your KH will lower your pH (no change in CO2).
Increasing your bubble rate will raise your CO2 and lower your pH.
Decreasing your bubble rate will lower your CO2 and raise your pH.
Baking soda will raise your KH and pH
A calcium carbonate source (e.g. crushed coral, aragonite) kept in the tank will (slowly) raise your KH, GH and pH.

A Note about Solenoids: You can use a solenoid on a timer to shut off the CO2 at night, when it's not needed. Rather than having to build up the CO2 again each morning, we just leave the CO2 on 24/7, as many folks do. If you turn off your CO2 at night, you can go about twice as long between cylinder recharges.

Fish and Oxygen levels: If you have both a large fish load and plant load, even the large amount of daytime O2 may not last the fish and plants through the night. Watch for fish gasping at the surface in the morning -- a sign of low O2. If in doubt, check your morning O2 level with a test kit. One solution is to run airstones at night. The surface agitation provides nighttime O2 (it also means that some CO2 will be lost from the water at night).

Equipment: Cylinder, regulator, bubble counter, reactor and tubing from Ultralife. Needle valve from Aquarium Landscapes. Water pump: Eheim mod. 1046 Hobby Pump