Probably fine to add after the fact. You could use a bucket and dilute it in portions first, then add to pool with stirring. I suspect the algae doesn't grow at all during the winter in Canada, so the algaecide isn't really a factor until the spring when temps rise enough that the algae starts to grow back. In fact, I've often wondered if the algaecide hasn't broken down by the point when it's really needed in the spring. PS, always go with polyquat 60 algaecide and not the cheap stuff that is largely ineffective and foams. I've bought my polyquat 60 on Amazon for a number of years, which has the best prices.