Hmm...well, if your water is merely dark/murky from lots of algae and there is no wildlife, frogs, snakes, bicycles, etc lurking in the bottom - then I would be tempted to hit it with plenty of bleach and 60% poly algaecide for about a week. (filter running 24/7, of course). Make sure you keep an eye on the filter pressure and backwash when it gets in the mid 20 psi range.

You'll want to get the chlorine level up as high as you possibly can without damaging the liner (if it's a vinyl liner pool), which would be about 15ppm. With lots of algae and goo in the pool it won't stay there long, so you'll need to pour in more jugs of bleach every day. For my pool (18x36), 27,500 gallons, one 3/4 gallon jug of bleach adds 1.6 ppm increase. So to get it up in the 10 to 15ppm range, I would have to add about 9 jugs of bleach.

(of course, if your pool is larger or smaller, you would have to adjust accordingly). I would also dump in 1 quart of 60% "poly" algaecide immediately. You will probably need at least 2-3 more quarts over the next week or so as well. The cheapest place to get Poly Algaecide is Leslie's Pool Supply. You can order from them online if you don't have a store near you.


Now...having said that, if you are uncertain as to what might be lurking in the water and/or you don't want the hassle of monitoring your filter and backwashing every day for the next week and so on, then I'd recommend getting a local pool maintenance company to come in. They will have a truck-mounted vacuum that they can clean your pool with (they'll probably have to vacuum several times). And for really dirty pools, they'll probably run your filter and use a temporary filter as well. Pool services are all over the place as far as price, so it's hard for me to estimate the cost....I'd set aside anywhere from $500 minimum to $1000 (or even a little more), depending on how much labor and chemicals are involved.

It's definitely not an impossible situation - it'll just take some time and lots of chlorine.