I Have 1 DYI co2 tank on my 55 gallon. 2 was to much for my crypts. they began to get leaf rot. The plants are recovering now.
I use an airstone to diffuse the co2. nothing more. I also have a good amount of surface aggitaition from my wet dry filters. My plants begin to pearl everytime the lights come on.
some plants will be damaged from an excess of co2. like my crypts. Even though my fish were fine, the co2 was to high for my crypts.
just wanted to let you know.
start out with one. then go to a 2nd one if needed.
hope this helps.