Ferrocement is the way to go, in my opinion. I really enjoy it. I will be making some ferrocement
water tanks of my own soon. I made a
root cellar of ferrocement and earthbags. Earthbags are another option, but ferrocement would probably be much quicker. My understanding is that the water will for a time be alkaline due to the concrete, but after a year or two, it won't affect the water whatsoever. I haven't looked too much into what is generally used to seal the tanks, or if it is always necessary. In my own tank, I will avoid sealers if at all possible. I would think that a rich mixture of portland cement on the interior and then painting the exterior would do the trick.