I have found that barley straw does a fantastic job of keeping algae out. I
bought a whole bale for $3.00 and a couple of those balls that hamsters like
to run in.

I put the algae in the balls and put the balls in the pond. It takes a
month or 6 weeks to work but one ball of barley straw keeps the pond algae
free for up to 3 months.

And buying it by the bale is a lot cheaper that when it is purchased from a
pond supply store or through the mail. They must be getting rich off of
barley straw judging from the prices they charge.

Haven't seen this mentioned lately so thought I'd toss it in.