Here is an example of two smaller scrubbers instead of one larger one; in this case it's two HOG1 scrubbers® on the wall of a saltwater sump. One obvious reason here is the lack of space, because a larger HOG1.3 or bigger might not fit into the compartments.
A second reason is continuous filtering: A scrubber filters more (absorbs more nutrients from the water) when it is full of growth, and thus when you clean/harvest it the filtering goes down for a few days until it grows back. But if you have a second scrubber that is not cleaned/harvested at the same time, this second scrubber continues filtering after the first one is cleaned. (All scrubbers must be cleaned/harvested though, to get the nutrients out).
A last reason is redundancy: if something happens to one of the scrubbers, you still have the other operating until you get the first one going again.
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