How do you get rid of black stains on the bottom of the above ground pool liner Ive tried scrubbing shocking algicide Nothing works?

Since you cannot remove the "stains" with the methods you have tried perhaps the stains are from too much acid or high acid levels. Are there also copper colored stains as well? Do you have a pool heater in the system? If so, then it is copper stains from an acidic pool water. The over acid conditions are stripping the copper from the heater copper tubing. k