Clean drinking water is a critical resource in an emergency.  Three days without clean drinking water then nothing else will be needed.

Rain water storage for human consumption requires blocking sunlight from the water to prevent algae growth.

Water for a survival garden can have algae.  Green water is fine for the garden.

Use a water filter to clean drinking water as it is used.  Filters remove bird poop, bacteria and more.

There is a treasure chest of information about collecting and storing rain water on Youtube.