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There's a difference between Filtration and Filters.
Filtration basically comes down to providing a safe / clean water as possible, by
biological filtering of ammo into nitrates. Mechanical filtration is removing of debris / nitrates (I personally consider PWC mechanical filtration).
You want plenty of filtration so your water stays pristine so I don't think you can over filtrate. However you can have too many filters running which at some point does not make the water any cleaner, but can create excess current.