The 'Askoll Co2 Natural System is a commercial product of genius, in my humble opinion.
The cost is € 20 and 'the method of trading strasposizione DIY (Do-It-Yourself) to produce carbon dioxide in the home, price reduced, and hence provide tropical aquarium co2. We talk about small aquariums ... for the larger ones is necessary to complete the "classic" with rechargeable bottle or not and its regulator.

In the system of ASKOLL however, has the problem of refills, that is, the sachets called "Activator" and "Stabilizer", ie, compounds with the main component certainly SUGAR and a bicarbonate and lyophilized YEAST the other. In fact, after 20-30 days ending 2 packets is initially effective, the bubbles of CO2 disappear and need "recharging" the bottle. The cost of recharging is limited, this is true, we talk about 10 € for 3 refills.

If instead we create our system components?
Just sugar and yeast, preferably in small cakes. Think of the cost ... we talk to about 0.20 € per charge ... :-)

The exact doses do not know, but ideally I could do 100 grams of sugar and 1/4 loaf of yeast.
Theoretically, one could also use the yeast to stabilize the production of carbon dioxide, pears this is recommended only if the water of the own tap has a KH of less than 10 ° ... and here in Rome are abundant around 15 ...

To test these doses'm trying it myself in my tank, so as to provide the best evidence and advice after a couple of tests "on the field." :-)