For this Instructable, the first I've done in a while, I will be showing you how to use an old liquor bottle to create a stylish, nearly free, CO2 gas generator for a planted aquarium.  The aquarium I'm doing this on is a small Fluval Spec V, this will work for larger aquariums however, just up the bottle size and CO2 ingredients accordingly.  This should run about 2 weeks before needing to mix up another batch of the CO2 creating ingredients.

What you need:
  1. 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)
  2. 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
  3. 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.
  4. 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.
  5. Super glue
  6. Sugar
  7. Baking Soda
  8. Bread yeast
Suggested but not required for this build, especially if you intend to keep fish with your planted tank, is a CO2 test kit.  These are cheap, you can pick one up at your local fish store, or you can pick one up on Amazon for 15 bucks.  http://amzn.com/B0052M9886  If your CO2 levels rise outside of safe levels, just cut back on the CO2 ingredients detailed in this instructable, use a control valve, or consider only running it during the day by installing a common air hose regulator available for a few cents at the fish store and cutting it off in the evening, or pull the hose from the bottle if you're around enough to manage that.  Plants consume CO2 and put off oxygen during the day offsetting the CO2 being put out, but at night they consume Oxygen... and the combined effect could kill off your fish during the night.  There is a lot of literature on the internet about these concepts, and of course a lot of opinions... and a basic understanding of the ecology of a planted aquarium should be obtained before introducing anything new that may kill your critters off.

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:
Java moss
Moss ball
Java fern
Dwarf sword grass
2 Porkchop rasboras
3 Blue rasboras
Zebra nerite snails
Ghost Shrimp

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

Pick your poison.  I chose a 750ml Old No.7 bottle, because I'm just classy like that.  The lid that came on this bottle was a regular screw top, which just won't do unless you want to play with expoxy and caulk to create an air tight seal, and likely make something pretty ugly.  A wine bottle cork would have worked just fine, but I found another bottle with a wine cork material insert with a black cap, even classier.

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.
It's pretty convenient that almost all booze is available in 750ml bottles. If anyone is interested/needs bottles, this summer I plan on emptying out a bunch of them.
I'll help you empty them out.

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