Sounds similar to my setup. Two 2L soda bottles fed into a manifold
with quarter turn valves so I can isolate the bottle I'm changing out.
My formula is a litle different, 3/4 cup sugar, 1/8tsp Fleischmanns and
6 cups of water. I have a mark right on the bottle so I don't have to
measure water anymore.

I get, like you said, about 3 weeks out of a charge. When I get close to
the end, a little swirling of the bottle seems to really get the
critters worked up and will send out more CO2 than the air coming out of
my air pump. When I add a new bottle I'll swirl the old one a bit to
repressurize the entire system. I wondered if adding a bit of air from
the pump would keep the reaction going longer, does yeast need air?
Would adding air reduce the efficency of the system?

What's the reason for the baking soda in your mix? I'm using the little
foil pouches of yeast, I carefully refold the pouch and tape it closed,
seems to work okay for several uses.

Now the question I've always wanted to ask. Is there any use for the
remnants of a bottle? Quite the fermented aroma as I pour it down the
drain so I know it's converting the sugar to alcohol. Maybe the yeast
eats gunk out of the pipes as I dump it? Always wondered.

Dave wrote:
I use Fleischmann's bread making yeast in a jar ... I can use the same jar
for at least a year and each batch is as good as the last. I use 2 cups
sugar, 1/4 tsp yeast and 1/2 tsp baking soda in a two quart (~ 2 liter)
plastic juice bottle. A batch will typically last about 3 weeks.

I actually have two bottles connected up via a "T" connector, and try to
swap out one every two weeks to have a consistent generation of CO2.
Sometimes I don't, though, in which case the mix will be nearly spent.
Because it usually takes about 24 hours for my new mix to get going (most
people report shorter times), I add some yeast to the existing mix to keep
it going until the new one is ready.

All of this means that I have a pretty worry-free, no-hassle DIY CO2 system.
Another thing that really makes things easy ... I have a pH controller which
operates an air pump to blow off excess CO2 when the pH gets too low.

For what it's worth ....

"Jon W" wrote in message
...

After adding a load of new plants to my 60cm tank a few weeks ago I
installed a yeast fuelled CO2 generator. This bubbled away happily for

about

10 days and the plants all seemed in good health, even my stoic old Java
fern started producing fresh green leaves.

The problems started when I tried to get a second batch going. The yeast
(which is Allinsons bread making yeast can be used for brewing according

to

the tin) has been kept in the fridge since it was opened. I have tried
everything to get a brew going but nothing seems to work. I have boiled

the

water and let it cool, left the yeast out for a few hours to let it warm

up,

made started cultures and added it to the body of the sugar solution.
Nothing has worked.

I'm sure the system isn't leaking at all and nothing has changed since I
made the first batch. The last thing I want is an inconsistent supply and
fluctuating pH. Does anybody have any advice or should I just admit that I


can't brew and leave my tank CO2-less? Unfortunately I can't afford a
pressurized system right now and all my new grow tube is growing is algae,
lots of it!