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A Simple, Inexpensive,  Electronically Controlled Do-It-Yourself

CO2 Injection System for the Planted Aquarium

Schematic diagram for the simple electronic controller shown in the drawing above, with an explanation of how it works.
Parts list for the electronic stuff. (Two or three 8-pin ICs and some passive components, available at Radio Shack.  That's it!)
I was frustrated by the limitations of the usual Do It Yourself CO2 generating method, in which yeast growing in a bottle of sugar water produces CO2 according to its age, temperature, and mood.  This simply doesn't allow enough control over the gas output.  If you have a small tank or soft water, uncontrolled CO2 input can cause a precipitous fall in pH when the yeast are feeling frisky.  If the yeast are old, cold, or lazy, you get lackluster performance and a rising pH.  The goal here was to build an enhanced version of the traditional DIY CO2 system that while still cheap and fairly simple, provides MUCH greater control over CO2 output, and that even allows for control by pH measuring equipment should one eventually choose to do so.  Having a reservoir of CO2 in the balloon means that much less vigilance over the fermentation process is required.  There are also no expensive and hard to find high pressure cylinders, regulators, needle valves, or solenoid valves.  And there are no trips to the welding supply shop for refills.
The Basic Setup
As shown in the diagram above, the system consists of the standard two liter soda pop bottle fermenter, connected to an air pump distribution manifold (from the fish store, marked "Valves", above) with air line tubing.  This manifold feeds both a Mylar balloon (the reservoir), and a small aquarium air pump.  The pump receives its AC power via the electronic Controller, which is built from a couple of readily available 555 timer ICs (or one 556 dual timer)  connected to a solid state relay.  Including the $8 DC adapter, this amounts to about ten dollars worth of parts, available at any Radio Shack store, as shown in the  schematic diagram.   This controller provides variable control over both the length of time the air pump runs and the interval between the pumping bursts.  In my current configuration, the pump sends half-second bursts of gas to a small gravel-filled PVC pipe "reactor" every 40 seconds.  This keeps my KH=2, 60 gallon tank at a pH 6.6, for a calculated CO2 concentration of about about 15 parts per million. (Unfortunately, knowing a little water chemistry like this seems to be necessary to do CO2 supplementation properly.  It's not very difficult, and it's all well explained at the 3 aquarium sites listed below.  Anyone thinking about CO2 enrichment is strongly encouraged to do a little reading about water, pH, buffering and the like first.)
The Balloon Reservoir
Since Mylar balloons leak Helium so slowly, I'm assuming that  CO2 molecules, being larger, will pass through even more slowly, and that air will be slow to pass into the balloon gas reservoir from the outside.  How much this diffusion of air and CO2 through the Mylar and attached tubing would affect the reservoir CO2 concentration over a long period of time is unknown, but it hasn't been a problem so far.  Needless to say, a rubber balloon won't do; it's simply too porous.   Mylar balloons seem to have some sort of valve mechanism (Mylar flaps that push shut under pressure) inside the balloon so that gas can flow in but not out. Be sure to place the tubing into the balloon far enough so it gets past this valve -- about 6 inches or so.  If the tubing is in far enough, air blown into the balloon will be come back out when the balloon is squeezed.  Seal the tubing in place with aquarium sealant, Shoe Goo, or whatever.
Pressure Release
It has been pointed out that if the pressure in the fermenter/balloon/lines gets too high, the CO2 would probably be driven through the pump and directly into the tank (i.e. you would be back to the standard DIY CO2 setup, in which the CO2 pressure controls the dosage.) To avoid this, some sort of pressure release mechanism is needed.  In my current setup, the tubing is just press-fit into holes in the pump enclosure. If the pressure from the fermenter should get too high (i.e. the balloon is completely full) the excess CO2 just leaks out around the press-fit lines. I have never actually had excess pressure build up, except when I've squeezed the balloon to see what would happen. This is because I use the standard 2 cups sugar, 1 tsp yeast and water mix, so the gas production is never too far ahead of the usage -- usually the balloon is about half full. A better solution would be to have a quarter-inch (5mm) hole in the pump enclosure (or the top of the fermenter) and cover it with a big ball bearing or a marble.  The fermenter/balloon side of the system requires almost no pressure at all (that's the pump's job), so loose tubing connections or a simple ball bearing release valve will work fine to prevent too much pressure from building up.


The Air Pump and Its Enclosure
Even though my air pump has a clearly visible intake hole and outlet port (leading me to originally believe that it was airtight), it had to be placed in a sealed container to keep air from leaking into the pump housing and mixing with the CO2. Apparently, the air leaks into the pump around the rubber base. I understand that this is typical of most diaphragm pumps, so be forewarned.  An alternative to placing the pump in a sealed container would be to tape or seal the pump bottom  to keep out the air, but this would make pump diaphragm changes difficult.  Putting the pump in a sealed container has an added advantage: it significantly reduces pump noise, especially if it is set on a balled-up piece of cloth to provide some mechanical isolation.

I imagine the carbonic acid produced by the CO2 is going to break down the pump diaphragm eventually, but so far (since December of 1998, so good.  A pump rebuild kit is readily available and doesn't cost much.  Given the extreme simplicity of a aquarium diaphragm air pump, I can think of no reason for it to be harmed by being switched on and off intermittently.

Putting the CO2 into the Tank: The Reactor
There are lots of ways of getting the CO2 into the water; please use whatever method appeals to you.  Feeding the gas into a power head or filter intake is probably the simplest method, and seems to work fine for lots of people.  The reactor configuration shown in the diagram above is very efficient and keeps most of the reactor outside of the tank, so the fish and I don't have to look at it.  A trickle of the water diverted from the canister filter return line passes down from the top of the reactor over the gravel within.  The CO2 bubbles up into the reactor through a hole in its outlet, 2 inches below the water level. This way there is very little back pressure on the CO2 in the supply tube coming from the pump. It should be noted that if your check valve isn't making a perfect, airtight seal, then water will want to climb up the supply tube when the pump is off -- there's no back pressure there to keep this from happening.  For this reason, I've chosen to feed the CO2 into the tank at the top, where the water pressure is low and the distance to the surface is small, so the pump doesn't have to expel a lot of water from the tube at the beginning of each pumping burst. No air stone is required at the outlet with this configuration, since water running over gravel provides lots of surface area.  Here's a diagram of a similar
reactor that was the inspiration for mine.

By the way, if your tank has a lot of surface agitation (from a spray bar, for example), the CO2 you're putting into the tank won't stay in there for long.  Consider returning the water to the tank in a manner that reduces surface agitation.

Input from External pH Measuring Equipment
The electronic controller can accept an on-off control input from a pH-measuring device with the addition of a single wire, as explained on the
schematic  page.  The system as shown above is working well enough right now, though, so I don't feel any burning need to add the additional circuitry.

Off at Night
In my setup, the AC power for the air pump comes from the timer that controls the lights, so CO2 delivery stops at lights out.  I find that this makes the pH much more stable in the tank overnight when CO2 uptake by the plants ceases.

Maintenance, Yeast Recipe
Once the system is assembled and the appropriate pumping interval and duration is set, the only maintenance required is recharging the fermenter bottle when the balloon starts to look a little flat.  (2 cups sugar, 1 teaspoon yeast, warm water.)  An advantage of this system is that since the quantity of CO2 going into the tank isn't dependent on the state of the fermenter, you don't have to pay very close attention to how things are going in the bottle.  You'll want to have a look at the pump every now and then to see how the diaphragm is holding up.

Alternative Implementations:  Pressurization & Valve,  Acid + Carbonate
An alternative system might be to put a one square foot piece of plywood and some bricks over the balloon to pressurize it, and then use a valve instead of the pump.  The same timer circuitry could control the valve's "open" duration and inter-burst interval (via a transistor instead of a Solid State Relay).  This would be a very low pressure system (i.e. << 15psi), since you'd have to place (literally) a ton of bricks on the square-foot plywood piece to get to the usual regulator output pressure of 15 psi (15psi*144 sq.in. = 1 ton!). The low pressure would require feeding the CO2 into the tank fairly close to the surface, just as it is in the system shown.  A system like this wouldn't require any more than 12 volts DC or so (to work the valve), so it wouldn't require doing anything with 120 volts.  This may be a more attractive way to go for people who aren't comfortable fooling with electricity coming right out of the wall. 

Another possibility that has been suggested is to use muriatic acid and carbonate material to generate the gas for the reservoir.

The idea here was to come up with something cheaper and simpler than the "standard" tank/regulator/solenoid valve/needle valve setup. The absence of a high pressure cylinder may make this system a little safer.  For someone who is comfortable with low-level electronic tinkering, it may be a viable option.  The whole setup was built with about $20 worth of parts from the hardware store and Radio Shack. 

My plants are growing like mad.  The fish swim happily amidst greenery and oxygen bubbles in an aquatic jungle.

Additional information about CO2 and planted tanks:

The Krib  Everything you ever wanted to know about freshwater aquariums.  A vast, wonderful resource. 

Digi-Key Electronics Home Page and Catalog.   The best mail-order electronics supplier.
People who are new to electronics, and those whose electronic skills have grown rusty, can learn a lot from Forrest Mims' Getting Started in Electronics.  Don't let it's simple format fool you, there's a lot of good information (as well as fun and instructive projects) there--probably enough to teach someone new to electronics how to build a simple project like this, anyway.  For the slightly more experienced or ambitious, Horowitz and Hill's The Art of Electronics ($50) is a wonderful once-over-lightly of the world of electronics theory and practice.

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