Did you read the front of the box or the instructions that came with the unit?
You may find it is designed for aquariums up 170liters.
I use them in my 165liter aquarium and the dose is 1 table everysecond day. I have a Co2 test kit but it does not register on this.
The holder contains 100ml of gas so not sure if you would need two and dose every second day or dose one tablet each day.
I experiment originally with the tablets in a coke bottle weighted in the tank and the hole was only big enough to get a pill in. The gas only lasted about 2 hours. In the proper container the gas last 6 hours. interesting!
My tank is an Ar850 so the water is sprayed out into the filter on top and cascades back into the tank so plenty of oppurtunity for the gase to come out. My plants do like the tablets they are looking good especially with the water consantly cascading. $13 for 20 pills = 40 days of treatment. I prefer this to DIY CO2 as it is more consistant and alot less maintainence.
Cheers I will tell you about my findings of soda stream one day and the troubles I had.