All plants in aquariums need co2 to live and thrive aquarium water is nearly always deficient in co2 natural co2 has already been removed from the water which flows out of our taps at the waterworks by means of aeration co2 fertilizers provide the necessary co2 to your aquarium in order to encourage healthy plant development and less algae something your fish will appreciate dennerle bio co2 with co2 control gel produces a virtually constant level of co2 for over 30 days each co2 bottle