It's possible, that 4 lbs of Cal-Hypo you dropped in spiked your FC to 50 ppm so in 3 days your algae and sun burned off 30 ppm.
5 gals may be enough, it may not be enough, depends on how bad the algae is, how much CYA you have, and how diligent you are with brushing, vacuuming and maintaining...