How much CO2 ppm for a 40 gal tank??

[rs1]

Is there a specific number I need to have in my 40 gal tank?

I've herd like 25 to 30 ppm is a good level.

 

[Fresh2o]

I think that range is regardless of tank size.
Generally speaking, the larger the tank, the more CO2 needed. You also need to factor in injection/diffusion method, tank circulation, and surface agitation.

 

[mbkemp]

30ppm is generally the target you want. It is fish dependent. If they can handle more, gsm. If not, dial it back. Move in increments, not big jumps