I use a CO2 diffusion system on my 109 liter tank. I found a Jungle "CO2 Fizz Factory" on a 50% table; $11.25. It is supper simple: a plastic container that is attached to the side of the aquarium with suction cups, plastic tubing to a plastic cup with a lid that has a plastic tubing attachment point.
About three times a week I drop an Alka-Seltzer tablet into the cup with 88 ml of water. The CO2 produced goes into the plastic container through the plastic tubing and is absorbed by the aquarium water.
I started using the Alka-Seltzer because the aquarium shop wanted $6.49 for a box of 16 replacement tables from Jungle. I can buy a 36 tablet box of Alka-Seltzer for $4.99. The Alka-Selter tablet produces about 250 ml of CO2 and the Jungle tablet 350 ml. Not a big difference.
This system is costing me about $10.00 a year. I do have to remember to put the tablet in three time a week.