They say 1 liter per gallon I think. But dont be afraid to give your plants more bubbles .. More bubbles wont hurt your plant but they will like it in the end .. The More the merrier. I use a big ass 12 port air pump 110 liter/minute for 4 plants and it pushes mass bubbles.i heard rena pumps are quiet so i was looking at this one http://www.planetrena.com/aquarium-air-pumps.html
the 400 series.
is there some kind of standard g/h or l/h or l/m per gallon of water that i can go by?