Why is the in ground pool chlorine registering as zero in spite of shocking it many times?

I'm having a similar problem - 2-3 days after shocking the pool and adding chlorine, the available free chlorine measures low or near zero. I've had the pool for 10 years and never had this problem before. I'm also getting excessive algae growth.

I took water to a professional pool store and they analyzed the chemistry for me. Everything was fine EXCEPT I had high levels of nitrates. Nitrates are heavenly food for algae and the chlorine reacts rapidly with everything going on in the pool.

I've been told to drain the pool and refill it. Apparently there is no other way to remove nitrates from the pool. I haven't done this yet, but it's on my schedule for this week.

Good luck!