What you need:
- 1 750ml bottle, emptied and completely rinsed out with fresh water (no soap, even trace amounts of soap can kill fish and may impede the CO2 production)
- Enough aquarium air hose to reach from the spot you want to place the bottle to the intake on your aquariums filter or outlet of the powerhead
- If the lid for your bottle is not already cork like or similarly air tight, a wine bottle cork... usually the same diameter of most liquor bottles will work fine.
- A backflow preventer, commonly referred to as a check valve, that is the correct diameter for aquarium air hose. Available for a dollar or two at most aquarium stores near where they sell air pumps and hose.
- Super glue
- Baking Soda
- Bread yeast
With this setup I haven't had any issues in this size tank, but it's always important to test water conditions with any aquarium on a regular basis.
In this aquarium, as an FYI, I have the following low light plants and critters:
Plants in Fluval Shrimp Substrate:
Dwarf sword grass
2 Porkchop rasboras
3 Blue rasboras
Zebra nerite snails
The rasboras will eventually outgrow this tank and the bio load will be too great, but I have other tanks to safely transfer them to when the time comes.
Step 1: Creating the Lid/valve
Take your chosen lid and the check valve to your drill press, a regular hand held drill will work fine too, just work out how you're going to hold the lid and drill through it safely, especially if you hastily consumed an entire bottle of your chosen poison to get going on this project. Choose a drill bit that is slighty smaller in diameter than the point of largest diameter on your check valve, most of which are tapered somewhat. The idea is to drill a hole you can shove the stem of the check valve into and get a tight seal in the cork like material.
Line up your lid/stopper so it's centered and drill all the way through. In this case, since I chose a stopper with a black cap on top, I made one pass all the way through the stopper, then flipped it over and used a larger bit to widen out the top of the hole just deep enough to get through the plastic. This was to allow my check valve to penetrate as deeply as possible into the cork like material (I say cork like because this one was that spongy plastic you often find nowadays in wine and liquor bottles, and is probably better for this purpose than cork anyhow).
After you've made the hole to your liking, put a small amount of super glue onto the outside of the check valve's stem (make sure you know which way the air flows through your check valve, you want the air flow to go from the bottle to the aquarium, so glue and insert accordingly), and work it into the hole. I wouldn't recommend putting glue in the hole first, you'll likely just block the hole and the airflow you'll need later. Work the stem around a bit so there is glue in contact with the stopper around it's circumference, but not towards the tip where it could block the hole.
Step 2: Prep Your Filter
First, unplug your filter and remove it from your aquarium. In the case of the Fluval Spec V, you just have to take off the powerhead output and the elbow joint on the hose and pull it out by the power cable. If you're not using a Fluval Spec V, and you're using either a back hanging filter or canister filter... adapt. The primary concept here is to get the bubbles to travel through the impeller fan that essentially any filter is going to have. I did the same thing on another aquarium with a back hanging waterfall style filter, and in that case no drilling or dissassembly was required, I just ran the hose and wedged it into the end of the intake tube and it worked perfectly.
On the Fluval Spec V, the face plate for the filter comes off very easily. Pop this off and take it to your drill. Pick a bit that is the same or slightly smaller diameter than the aquarium airhose. Mark the faceplate with a dot centered directly over the little impeller fan inside the unit, so that when the bubbles come out they get sucked directly into the fan, get chopped up into micro bubbles and spat out into the tank. Drill your hole on the marked spot.
Once your hole is drilled, your airhose should slip into it and stay put, use a little super glue if you need to. Do NOT situate the hose so deep that it interferes with the fan or the water flow into it... just barely breach the little faceplate. Snap the faceplate back on and use a tiny zip tie to secure the airline hose to the outlet port on the filter body, without restricting that hose in any way or crimping or pinching off your airline hose. Put your filter back into the aquarium, and hook the elbow joint and outflow valve back up the way you found it. Run your airline hose to the spot you're going to stick your CO2 bottle.
Note to Instructables Contest Judges: I am adept at AutoCAD and Google Sketchup, and if I had a 3d printer I would make a new faceplate for this build specifically adpated to the purpose, and I would have a huge smile on my face, and I love you.
Step 3: Make a Batch of CO2
While that is sitting fill up your empty and well rinsed bottle about 3/4 of the way with warm water. Using a funnel add 1 cup of sugar, really any sugar will do but the white stuff is the cheapest, and we're feeding yeast here. Add 1/2 teaspoon of baking soda (this keeps the pH high, good for the yeast). Using another undrilled cork, or if your thumb is big enough, plug up the end of the bottle and shake until all the sugar and baking soda is dissolved and not settling on the bottom.
Once approximately 15 minutes have passed since you added the yeast to your sugar water mixture, poor that mixture into your bottle with the sugar/water/baking soda mixture. There is no need to shake or mix this, and that may in fact harm the yeast, just pour it in.
Step 4: The Hook Up
Step 5: Troubleshooting
If you have fish in your planted tank, they will probably be fine, but in some cases CO2 absorbtion in the water can get high enough to essentially suffocate your fish. As any aquarium geek will tell you, watching your fish and learning how they behave is a critical part of taking care of fish. This is no different. As the CO2 levels rise, you will have plenty of time over days to notice the behavior of your fish change if the CO2 absorbtion is getting too high and your plants can't keep up, and the gas isn't aspirated fast enough into the atmosphere. If your fish start hanging out at the surface more than usual, gulping for air at the top, or hanging out next to the powerhead or inflow and they weren't previously, you might be in danger of suffocating them with CO2. If that's the case, you don't need to get rid of your CO2 system, you need to get more oxygen into the water. Usually something that breaks up the surface tension of the water will suffice, like pointing your powerhead so it lightly breaks the surface tension of the water... or adding an airstone will do the trick.