In short, there is no short-cut to getting rid of algae. You have to kill it and kill it all, and the ONLY way you do this is by raising your FC level. Bleach is the same as Liquid Chlorine, but pool stores sometimes sell LC at 12-14% levels--so 1 gallon is the same as 2 gals of 6% Bleach. You can use LC as well. You can use the Bleach Calculator to figure how much you need.

But there's the rule of thumb: 1 gallon of bleach added to 10,000 gallons of water adds EXACTLY its concentration. So 1 gal of 6% adds 6ppm, 1 gal of 5.25% adds 5.25ppm and 1 gal of 12.5% adds 12.5 ppm to 10,000 gallons.

You need to add two gallons of any of these to 20,000 gallons to get the same increase in ppm.

So say you have 20,000 gallons in your pool and your ppm is 3. You need a minimum of 17ppm. So if you add 3 gallons of 12.5%, you'll get an increase of 19.5ppm. Or you'll need 6 gallons of 6% bleach for the same effect (increase of 18ppm).

It's not too hard to figure.