, 6.4sccm is only for the first hour, if keep pumping co2 at 6.4 sccm after the first hour in the 10 gallon tank, the fish are probably floating up side down very soon, lol.
jeff's point is 10lb co2 tank will last a very long time for a 2.5 gallon tank.
to be exact, if co2 injection is 10 hours a day , first hour at (6.4 X 10G/2.5G =) 1.6 sccm co2 flow rate will increase the co2 concentration above 20 ppm, the total volume of co2 in first hours is 1.6 X 60 = 90CC, then the next 9 hours co2 flow rate will keep at 1/4 of 1.6sccm(assume this is the co2 lost rate), to replenish the lost of co2 so the concentration will stay above 20ppm but lower than 30ppm.
1.6sccm X 1/4 X 60minutes X 9hours = 216 cc
90cc + 216cc = 306cc
the total volume of co2 required for the 2.5 gallon tank in a day is 306cc.
10lb co2 tank has about 2430 liter of co2, then 2430liter X 1000(CC/liter) / 306(CC/day) = 7941 day...
about 20 years.