Well it isn't MY idea!! I guess there are algae spores all over and they get in the pool. Certainly there aren't any right after the spring chlorine shock. But chlorine is volatile and dissipates from the water which is why people with chlorinated pools need to test daily and add chlorine as necessary. The Pristine Blue chemicals also kill algae but sometimes it gets overpowered when the temps are high (good for algae growth). With Pristine Blue, you take a water sample to the pool store every 2 weeks where they test it (free) and you get a printout of how much of each chemical to add.Algae goes in pools?