adding baking soda buffers the mixture so that it doesn't run out that fast and also to control the bubbling rate it'll not affect your tank's kh. i personally think it's advisable to add so that the mixture wont go haywire(ie produce a lot of co2 in the first few days then after that the mixture just dies
)
hmmm but when the water comes out from the sump it'll be gone since it's sort of like a waterfall(?) at the output area? since there's surface agitation the co2 will be gone... perhaps it's more advisable to have another powerhead at a more remote area in the tank where there's not that much surface agitation so as to prevent co2 loss
