Ok, here's what I've noticed after doing a do it yourself co2 injector
set up.

What I did was get a 2 litre pop bottle, drill a hole in the cap, feed
in some tubing and put a bubble diffuser on the other end. Into the
bottle I added 6 cups of warm water, 3/4 cups of sugar, and a teaspoon
of yeast. Put my hand over the cover, shook the bejesus out of it,
screwed the cap on tightly, and moved the tube through the cap so that
it was suspended in the air above yeasty solution. I set the diffuser
into the kitchen sink before checking things out in the aquarium for
safety's sake.

Round one: no bubbles. :O(
Round two: increased the yeast to an entire package (about 1
tablespoon). One hour later, VAVOOM! Bubbles in the sink! Took the
contraption to the tank, put in the difuser, waited. Worked for 3
days. Then went poof and died, dumped the contents, waited until
payday to go buy more yeast and try again. I think the reason the
diffuser didn't work so well first time around, was because I used too
much yeast and it fermented/played out too quickly.
Round three: changed the recipe slightly. Added 3/4 cups sugar, 6
cups water, and only 1/2 package of yeast (about 1/2 tablespoon).
Waited one hour...bubbles in the sink....TO THE AQUARIUM, Batman! The
thing's been sending a clumpage of bubbles out on average about once
every 7-10 seconds for 7 days now. I've noticed that the java ferns
have suddenly started sending off little brown thingies on the tips of
their leaves that have since become baby java ferns. Other plants
have grown slightly more rapidly, colour is definitely deeper, and I
get bubbles! Yes, bubbles! My plants are pearling! I keep an eye on
the diffuser, when the bubbling stops, time to make a new batch. I've
heard that some people will get a bottle ready and on the go for when
the bottle being used starts to peter out. Dunno...

I'm new hey? Fish I got a handle on. The added co2 has increased the
acidity of the tank just slightly, but nothing the fish don't seem to
mind. Watch the reactor! I've heard horror stories about yeast doin
stuff that flows into the tank, tank water flowing back up the
hose...I've just been using basic common sense, and my visual senses.

It's cheap, it probably isn't the best way to introduce co2 into an
aquarium set up, but if you're just looking to experiment and "play"
(the fun part of the hobby for me), give it a try.

I even think the krib has some stuff on how to make them, problems
people have encountered, stuff like that. And I'm sure there are
people here who've had experience with both the big high tech set up
and the coke bottle affair (although I prefer Pepsi).

Cheers!

Maq
aka Pete