We had one, go to the local pool shop, they should hopefully have the pool on their records as they need to know how much water it holds. They will work out what you need to do to clear it up. It's cheaper to keep it clean rather than let it go green over winter. Does it have a cover? That will help keep out the muck and leaves in autumn and winter. Our filter was very expensive to run so see if you have cheaper rates at night and it should only be on for 2 hours a day ATM. Ps I wouldn't trust the old owners if they left it in such a bad state! A good pool shop will come and show you if you can't work it out.