Answer:

Get any large debris and sediment as you can out manualy, you may need to scrub the walls and floors to destroy stubborn algae and other growth. You can also vacuum, but make sure you vacuum to waste so it does not recirculate it back into the pool. The next step will be to balance the water in the following order 1. hardness 2. total alkalinity 3. pH. Once the balance is acheived, you would need to add a metal removing agent if there is any metal typically present in your source water before you add chlorine as once you add chlorine, the metals may oxidize causing further cloudy water and staining. Then you are ready to add chlorine, following the dosage chart provided on the chemicals. You will need to shock the pool first to establish a chlorine level, then keep it going with whatever chlorine you will use. Again, test and balance to make sure you have acheived the proper levels. Then if black coloration or algae persist, you may want to consider the addition of a algaecide specifically for black algae as this type of algae can be stubborn and even chlorine resistant.