What is this? Algae?

[Fataltruth]

For the past couple months I have noticed this on the bottom of my above ground pool. I believe the liner is vinyl.

My chemistry ( taken weekly from a Taylor kit)
FC: 9
TA: 70
CYA: 50
PH: 7.4
CA: 70

It is a 12000 gallon 24 ft round pool.

My weekly strategy is to backwash my sand filter, vacuum bi weekly, brush weekly, and test chemistry weekly. I only use bleach and baking soda, cya, and acid to control my pool chemistry.

Things I notice.
- after brushing my pool the “marks” disappear but then slowly reappears after several hours.
- I thought it was black algae ( I know it’s rare in vinyl) so I added 4x slam and scrubbed my pool with scrubbers tied to my feet, and ran the pump for a week. Didn’t seem to change much.

 

[Mdragger88]

It looks like dead algae-
If your fc has ever fallen below minimum for your cya it is most likely algae.
FC/CYA Levels

This seems like the most logical explanation if you’re only testing fc once a week.
The pool eats fc every day.
Making up for what you’re lacking on the weekend really doesn’t cut it although it may be helping to keep it from becoming a swamp.
We don’t recommend “ 4x slam” or some arbitrary amount we recommend doing the SLAM Process to completion.

 

[kellyfair]

Looks like algae to me too. Mdragger has you with the SLAM recommendation. I would also add that if you can, testing chlorine more frequently will help you keep chlorine from dropping too low. Also, your filter may not need to be backwashed weekly. Make note of your clean pressure then backwash when it rises 25%. Can save some water and CYA if you find you don’t need to backwash frequently. Also, sand filters work a bit better when slightly dirty.

Lastly, I really need to see a photo of the scrubber feet!