If bacteria convert CYA into ammonia, then for every 10 ppm CYA that gets converted it creates 3 ppm ammonia that requires at least 23 ppm FC cumulatively added to get rid of (technical details in this post). So it doesn't take very high CYA levels to have a huge chlorine demand. My own personal experience described in this thread took around 56 ppm FC cumulatively added due to a drop in CYA of only around 20-25 ppm. So let's hope that's not the situation in this pool. When algae is present, as in this pool, the algae take up a lot of the ammonia (they use it as a source of nitrogen) which fortunately makes it easier to remove as one can catch the algae in the filter and then backwash/clean the filter.