I have been working on a few commercial pools with black algae for the past few months. It is the classic example of why the 3 "tions" are needed. (circulation, filtration, and sterilization) To get rid of stubborn black algae I have been diving, with a chlorine puck and a "dental" pick (do not dig just scrape),and a strong bristle scrubbing brush. Use in this order
1. rough rub with chlorine tab (this weakens the protective barrier)
2. scrape with pick (only remove what is on the surface, and only use the pick to poke in holes where you cant scrape)
3. firmly brush
4. push hard scrape with chlorine tab. (this forces chlorine into the roots)
Here is what I have found that has worked so far to help control black algae. Sorry this is long and not simple....
lack of circulation. This allows the black algae strand to "root" itself without the threat of washing away, and creates a significantly lower chemical spread at the deep end.
water returns all facing up or sideways
turn sidewall returns at a downward angle to increase deeper water circulation.
not enough water pressure in returns to make it to the bottom of the pool
A booster pump fed "robotic" cleaner will distribute more circulation on the bottom of the pool (not inexpensive)
"robotic" cleaner not running long enough to be effective (many will set the cleaner to run just long enough to clean average debris)
change timer tabs to allow the cleaner to run longer (possibly 1 hour after pool pump turns on and 1 hour before pool pump turns off)
no pressure from bottom returns
Equalize bottom returns by closing the working returns and forcing water to the non working returns (requires extensive time on the bottom of the pool) Once the returns all work, only open them back up enough to feel water flow 360 degrees around the return. (weak water pressure is better than none)
Once again, these are just my findings and are NOT the only causes and solutions to this problem