Quote Originally Posted by Monks-in-Miami View Post
I've noticed that the rain water collected in my barrel is sometimes green. Not sure if this green -- which I assume is due to algae -- affects the plants in anyway when I water them. Should/can I add some physan just to keep the algae down?
Algae, per se, does not affect your plants. One might argue that dying organic matter within the potting medium is bad, but frankly, I doubt that there's enough to be of any consequence. I used to add Physan to my RO tank for that purpose, but some plants (notably dendrobiums and thin-leaved plants like coelogyne, for example) don't like it.

If you can fins a way to block the light getting to the barrel contents, that'll take care of it.