Hello, welcome to pond boss.

You say you drain your pond every couple of weeks and scrub the walls? that may be the first time I have ever heard of anybody doing that, at 300 gallons it shouldn't be a lot of scrubbing, is this a pond or a small pool?
Its probably going to be pretty hard to treat the water with anything to reduce the algae with it rotating out so fast.

Hopefully the professionals will be along on this site to suggest a better solution to your issue.

Good Luck!

All the really good ideas I've ever had came to me while I was milking a cow.