How can I collect rainwater without it going stagnant? My waterbutt always
goes green (the water, not the butt!) and has all sorts of nasty insects
living in it, not nice...!

Does it need to be out of the sun / lidded? Or can I add steriliser tablets,
or something?

Any other good ideas how to collect clean rainwater for indoor houseplant
use?

Many thanks!!
(sorry if I sound ignorant, I'm a bit of a newbie)