How much water can I safely drain from my pool?

[rfera]

Hi, I have an above ground, 18' round pool that didn't get opened at all last year. My sand filter canister cracked (I guess I didn't get all of the water out before winter) and I couldn't afford to replace it. I was just given a cartridge filter and pump and would like to get the pool opened, finally!

My question: since this pool is now a green pond, how much water can I safely drain out? I assume it will be much easier to clean up a less-pond-like pool than what I have now; I just don't want to create more problems for myself, structurally.

Any advice is welcome!