i was using 5l bottle last time also. I used half kg of sugar, 1 big soup spoon of yeast and 1/3 tea spoon of soda powder. It worked for 8-10 days.

you can prevent the mixture goes into the tank by connect the CO2 bottle to another bottle with 2/3 water acting as burble counter. The water level must be higher then the hose from CO2 bottle. So that, you can see the CO2 burble coming out from the hose. And another hose from the burble counter to the reactor/refusser just like the hose connected to the cap of CO2 bottle.