Within the last week or two, I've seen several pools that had no problems all summer, suddenly show significantly lower FC levels. This got me thinking about how this could happen.
I'm thinking that these pools have had small amounts of algae growing all along, but in such small amounts that the SWG was able to maintain significant FC levels. There would have to be an equilibrium between the amount of FC generated and the amount consumed fighting algae. In many of these pools, the FC is up around 7 or 8 with CYA somewhere around 60. If the right conditions present themselves, the algae would have opportunity to shift the equilibrium, resulting in a sudden drop in FC and if not corrected, a full algae bloom.
If my thinking is correct, I could expect the same pool to maintain the same FC levels at a lower SWG setting if there were no algae living in the pool, since none of the FC generated would be consumed maintaining the equilibrium with the algae.
This would also mean that the shift in equilibrium could go the other way. If instead of conditions favoring the algae, they could favor the FC resulting in a slight shift allowing for greater retention of FC resulting in higher FC levels that should more rapidly kill algae.
So this all seems reasonable to me, but it is all based on the assumption that algae can be killed below shock levels. So is it possible to kill algae without raising FC to shock levels?