Don t leave the chlorine on the spot for too long as it may bleach the liner.
Black algae vinyl pool liner.
These micro organisms develop colonies behind the liner and left unchecked eventually will permeate through to the printed side appearing as dark gray or black splotches.
What differentiates it from algae is that it s more of a parasitic type organism says terry arko water specialist for seaklear pool spa products based in bothell wash.
We think the bacteria can be washed into your pool area by flooding rains or when a septic tank overflows.
Black algae in pools is similar to that which can be found in between bathroom shower tiles and on silicone seams near the bathtub.
Black algae in a vinyl pool is possible but not very likely.
Also black algae seldom forms in pools with fiberglass or vinyl liners.
As long as you are losing fc overnight you still have something organic in the water that needs to be taken care of so you should continue shocking.
Black algae is almost unheard of in vinyl liner pools.
For a semi bad part.
If you have black or grey cloud like stains on the floor of your in ground vinyl liner they are most likely caused by bacteria that is living in the ground under your pool.
These organisms are tough and have a thick coat to resist chlorine effects.
5 10 minutes max.
Your marks sound like organic stains that will fade away slowly as long as you keep chlorine in the pool.
It s more at home in concrete plaster or gunite pools because they re porous surfaces the organism can really latch onto and even grow roots past the surface.
Black algae is a thread like growth that develops on rough surfaces in swimming pools.
To remove them start with a rigorous brushing using a nylon brush.
A few other ways to recognize black algae in your pool.
Black algae will dot the liner in black spots.
The resistant coating will become weak and break.
You can also use about 1 2 lb.
Of calcium hypochlorite granular chlorine in an old sock to put on a spot.
Alot of people initially treat it chemically as a algae growth major shock treatment in addition to a special algaecide.
It appears as dark black spots on the walls and floor of the pool and is slippery to the touch.
Over the last 12 yrs i have probably looked at over 100 inground vinyl pools with this black mold growth under the liner.