Generally speaking, it is a good idea to use an algaecide, such as PolyQuat 60, when doing the metal treatment that by necessity has Free Chlorine (FC) levels go to zero during the ascorbic acid treatment. Otherwise, algae can grow. This is why this sticky refers to its use.

In fact, one easy way to get the FC level lower for the Ascorbic Acid treatment is to add a lot of PolyQuat 60 since high doses will use up chlorine.

Richard