Hi Lily and welcome to GoY. Is this a new pond? If so you can expect there to be algae as it establishes itself. Plants won't feed on the algae, neither will the fish which will be causing part of the problem.

I remember Charlie Dimmock saying that a pond will clear after its second spring. I only have a half barrel, but I have had to refill it a couple of times,and have generally found this to be the case. If, as Moongrower says, yours is a new pond, you may just have to wait for it to establish a natural balance.

Don't give in to the temptation to change the water or you will have to start waiting all over again...

I recently discovered that barley straw is only effective for six months and takes a month to start working.

Didn't make any difference to our pond at all. Maybe there wasn't enough of it.

Needless to say, I didn't refill the barrel in order to solve this problem. As Steragram says, if you refill it, the waiting starts all over again!

The green water is harmless. It's full of oxygenating algae. As a matter of fact if you placed an algicide in the pond it might go quickly anoxic and your fish would die. You must get used to the idea that it is a pond that is developing a complex and balanced ecosystem and not a bathtub.