With a sand filter, using a clarifier can help. Polyquat (info page at PoolSolutions) is BOTH an algaecide and a very effective clarifier, that doesn't have some of the overdosing problems some clarifier only products have. Use it, if you can.

Otherwise, you can buy a clarifier locally; it won't hurt, and it may help. BUT there's a great BIG caution here:
1. Do NOT exceed the recommended dose or frequency . . . EVER.
2. Do NOT use a clarifier if you are thinking about using a flocculant. One or the other; not both.

Some standard clarifiers will actually work in reverse, if you overdose!