Try useing 10oz of aluma-brite to 2 gallon of water to brush the stacks,they can take it stronger if the 10oz doesn't clean them up.
If the tanks are polished do not let the aluma-brite drip down on them when brushing the stacks.If polished tanks dip the brush in the acid and shack it a little as to not have a fully loaded brush as to drip.