rambling DIY CO2 notes, with reactor plans (long)

I recently put together a yeast CO2 system based on Thomas Narten's
great article. (See
There are a few details I noticed which I thought people might find
useful. I also ended up building a CO2 reactor, which design is
detailed here. I will assume the reader has read Narten's article, so
I won't explain the whole process.

A few tips about getting the CO2 out of the 2-liter bottle and into
airline tubing: #3 or #4 stoppers both fit 2-liter bottles. If your
stopper has no hole, you can drill a hole in the stopper yourself if
you hold it with pliers to make it more rigid. Copper tubing can be
cut by wire cutters and/or a file; you use the cutters to score around
the outside of the tubing before snapping it. This takes a steady
hand, but it works, and it doesn't require you have a tubing/pipe
cutter on hand, which I don't. Stoppers are around $.50, and copper
tubing is around $.70/ft, at the store I went, so this is not an
expensive thing to set up.

Setting up the stopper - and remembering to buy yeast at the grocery -
were the hardest parts of getting the injector set up. This method is
very easy to get going!

I also put a check valve in the airline tubing; based on various
postings I'm not sure if it would prevent a catastrophic siphon, but
who knows, it might, and it was only a buck. I waited a few hours
after assembling the yeast/sugar mixture before hooking up the tubing
anyway. (I might move the 2-liter to a shelf at water level anyway.)

Nicholas Plummer reports in Narten's article that users of Hagen
Fluval 203 filters can experience the impeller rattling if they use
the filter to shred CO2 bubbles. I can confirm this, at least with my
Fluval 303 (loaded with, in order, ceramic rings, empty, foam). This
woke me up at around 3am one morning. It's easily remediable by the
usual "fiddle with the valves on the intake/outflow lines until it
goes away" remedy Fluval owners are used to. (These filters make a lot
of noise if enough gas gets trapped in the impeller chamber. I'm
thinking of chucking it and getting an Eheim.)

So what better time than 3am to make a CO2 reactor? I made one from
what was on hand. Following is a description of the reactor, which is
fairly simple, as it does not use a pump to push water and CO2 past
each other. The materials:

- 1.5L bottle from Volvic Lemon-Lime Water. Yummy stuff, around $1.50,
bottle is transparent and label is easily removed.
- Plastic cable ties. $3 for a whole bunch of these.
- Around a quart of bio-balls, bought a few months ago for this
eventuality. These are expensive: I think I paid $15/gallon or
so. Aquarists on a shoestring can use some other medium; I've seen
filter floss and marbles both suggested.
- Airline tubing and an airstone

The Volvic bottle is a rounded square as seen from the top. I cut the
top off the bottle, drilled various holes in it, and filled it most of
the way full with bio-balls. I then cut small slits in the sides with
a sharp knife and ran cable ties through these slits, connecting the
ties to each other in a rectanglular loop. As viewed from the top or

       /--------\           /--------\ 
      /          \         /          \
      |          |       | |          | 
      |          |       O-+----------+-\
      |          |   ->  | |          | |
      |          |       | |          | |
      |          |       \-+----------+-O
      |          |         |          | |
      \          /         \          /
       \--------/           \--------/ 

These cable ties are placed so they will hold the bio-balls in place,
making it possible to handle the reactor without the bio-balls falling
out. As viewed from the side, the apparatus looks like this:

         v-- holes in top to allow undissolved CO2 to escape

       / ------ \ 
      /  O O O O \
      | O O O O  |
         O O O O    <- holes in sides to allow water admixture
      | O O O O  |     (these are important since there is no pumping
         O O O O       action to mix the CO2-enriched water with the
      | bioballs |     aquarium water)
        O O O O   
      |  O O O O |
        O O O O   
      |  O O O O |
      | O O O O  |
      |  O O O O |
      | O O O O  |
      +----------+  <- cable ties holding in the guts
      |          |
      |  (===)----- <- airline tubing through hole to airstone
      |          |
      |          |
      |          |

The bottle holds roughly 16 of the size of bio-balls I have.
Incidentally, these are far larger than optimal; I think tiny
bio-balls (under an inch in diameter) would be more efficient at
slowing, trapping, and breaking up bubbles. Or maybe gravel.

The "spare" length of bottle on the bottom allows for implantation in
the gravel, to prevent the reactor from going adrift. I found the
bio-balls to be slightly buoyant, which surprised me, and I figured
any air or CO2 trapped in them could only make things worse.

Next time I'm going to try using a hole punch and/or a pocketknife
and/or a soldering iron instead of a power drill for making holes in
the plastic, as drilling thin plastic generates really ragged holes.

The decision to use cable ties seems to have been a good one. They're
so cheap as to be nearly free, they hold securely, they're easily
removed/replaced for cleaning, and they're strong - stronger than the
plastic of which the bottle is made. I thought about putting the top
of the bottle inside the apparatus to hold everything in place -
imagine cutting the top off the bottle, inverting the top, and
sticking it back on/in the bottle. But I decided this would be a waste
of space inside the reactor, as there would be odd shaped spaces where
bio-balls would not fit. So depending on your choice of reactor media
this might be useful.

Right now I don't have the airstone fixed to anything in the reactor.
I decided to leave it this way for ease of maintenance. There's not a
lot of room for it to wander around anyway; it rests on the gravel
with the first layer of bio-balls right above it.

This reactor is not pretty. I now have a funky looking transparent
quart-sized bottle stuck in the gravel in a back corner of my
aquarium, with red bio-balls inside. It almost makes me want a sump. I
will have to replant a bit to cover it up.

Well, that's about it. The gizmo hasn't been in place long enough for
me to report on its efficiency at dissolving CO2, but based on the
quantity of bubbles coming out of the top of the reactor, I wager this
efficiency is low. Time to investigate small pumps.

Based on my water values prior to the CO2 system - nearly no dissolved
carbon dioxide, pH around 7.8, KH around 40ppm raised to around 70ppm
with NaHCO3 - and on various net postings, I have high hopes for the
effects of CO2 injection in this tank. I figure with 120W of tritons
over the tank, CO2 was the next step. Indeed, after a couple of days
running, the pH has stabilized around 6.9 (6.7-6.8 in the morning),
with dissolved CO2 reading as expected around 15-20ppm.

Next step: researching iron! (I have crappy substrate.)