Rain Water / Refreshing Pond Water

[GBBUDD]

Rain water is fine. I believe the runoff to be responsible for your algae bloom. Gives it plenty of nutrients in which to thrive.
Go ahead and kill most of it pre-pond (I.e. in the filtration reservoir or piping) before it takes a stronghold. UV is your friend. No chemicals and WILL kill algae.

UV KILLS FLOATING GREEN ALGAE , but those cells then become food for the next generation of algae cells . This becomes a endless battle . you want to REMOVE the excess nutrients . be it by the use of a sieve , sand filter, Bogs AND PLANTS or rdf. Water changes can help if you are real bad.