black algae and chlorine

Hello all, I am having some trouble I was hoping you can help with. I have a underground pool with liner. About a week ago, I noticed black spots in numerous locations of the pool wall and floor. I scrubbed and added 4 lbs of shock (20k gal pool). Two days later, black spots came back, chlorine level was zero. (I'm using the type of kit that you add drops to a little sample). I added 5 lbs of shock and vacuumed again. Yesterday, black spots came back, chlorine level again zero. I added 5 lbs again and vacuumed. Today, black spots not back but chlorine is gone. A just ordered black algae treatment. So, is the black algae eating my chlorine? What should I do? Thanks a million.

The black algae probably is eating your chlorine, but just for reference, could you please post a set of test results? FC, TC or CC, pH, TA, CYA, and what you've used for chlorination?

For now, the best thing you can do is get it up to shock level and keep it there, not allowing the Cl to yo=yo up and down. I'll let others chime in about how well the black algae treatments work. Black algae is very stubborn and resilient, so it's not going to be an easy fight!

Janet

Hi Joe and welcome! I moved your question into the algae section of the forum instead of the Forum Q & A section which is for questions about the forum itself.

We really need some current water testing results to be able to help. We need:
pH, alk (TA), CYA, and at least two of the following -- FC (free chlorine), CC (combined chlorine), TC (total chlorine.)

Also, tell us exactly what you have added --- meaning ingredients, not just "shock" or "algaecide." Where is the chlorine going? It is being used up killing algae.

Repost with the needed information and somebody here can help you. Also, tell us what type/size pump and filter you have.

EDIT --- Looks like Janet was posting while I was typing! And, we both want the same info.