Water purification tablets ? Do you use enough volume of water to justify a large container ? A large slow flowing reserve of water would be subject to alge forming along with other bugs and beasties with out treatment.
If you need a large amount consider some basic filtration and some treatment.
We use a 200 ltr drum to ensure good turn round without treatment. Having said that we do have 4000ltr standing water in IBC's that has been treated, multi use , static caravan anchor/emergency water supply/ fire pump supply.