Can you get a feel for what the smell might be? Is it musty, bad eggs, rotting vegetation or worse? Is the water cloudy or clear? Does the water itself smell or just the cisterna?
Household bleach which is labelled as safe for drinking water is fast dispersing and very cheap. the stuff we get from Suma is safe but from Lidl is marked as not suitable. Check the bottle.
Pool shops sell an enzyme based product which, like biological washing power, eats up any organic matter. If it's safe for pools then it's safe for people if you follow the instructions.