I used to do the DIY yeast CO2 injection directly into the uplift tube of
my
HOB filter for my 39 gal tank. With that setup I was able to achieve
about
18 ppm CO2 saturation. I found that I could get a good 8 days of
production
from a 64 oz mixture (juice bottle) and so I would change the mixture out
every weekend with a new batch that was started 24 hours prior (new batch
usually mixed up on Friday night). That way, every Sat or Sun the
aquarium
got a new charged up batch of CO2 that *could* last 8 days.

Thanks for the replies everyone. How long did you inject into your
filter input tube? I've heard that the bubbles can play havoc with the
filters propellers and rubber fittings, plus they make more noise. But that
most mostly with injecting directly into the input and skipping the tube (at
least, that's what I remember reading). That was one of my first instincts,
but I'm cautious about damaging the filters. On the other hand, did you
notice if the increased suction from the filter input had any negative
effects (e.g., did it ever suck any fluids from the CO2 canister when its
pressure dropped towards the end)? Cheers,

Tony

p.s.- I've been calculating my water changes based on tank size- is that
correct or should I calculate the % based on the actual volume of water in
my tank (e.g., my 55 gallon tank probably only has about 45ish gallons of
water, so a 30% change is actually a 35% change, etc., meaning I'm changing
35-40% of the water each week-closer to Tom's suggestion).