We think the bacteria can be washed into your pool area by flooding rains or when a septic tank overflows.
Black algae vinyl pool liner.
Black algae in pools is similar to that which can be found in between bathroom shower tiles and on silicone seams near the bathtub.
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.
Don t leave the chlorine on the spot for too long as it may bleach the liner.
5 10 minutes max.
Black algae is a thread like growth that develops on rough surfaces in swimming pools.
For a semi bad part.
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 will dot the liner in black spots.
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.
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.
Bacteria sometimes show up years after a pool has been installed.
Alot of people initially treat it chemically as a algae growth major shock treatment in addition to a special algaecide.
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.
You can also use about 1 2 lb.
The problem area will lighten up for 3 5 days then come back to its original color size.
Of calcium hypochlorite granular chlorine in an old sock to put on a spot.
It appears as dark black spots on the walls and floor of the pool and is slippery to the touch.
The resistant coating will become weak and break.
To remove them start with a rigorous brushing using a nylon brush.
Over the last 12 yrs i have probably looked at over 100 inground vinyl pools with this black mold growth under the liner.
A few other ways to recognize black algae in your pool.
Also black algae seldom forms in pools with fiberglass or vinyl liners.
Your marks sound like organic stains that will fade away slowly as long as you keep chlorine in the pool.