Overstocking is not merely from a biological filtration point of view, it's more with respect to the space that marine fish need to remain stress free. It's not like fresh water tanks where you can place great numbers in small tanks.
Some fish like tangs especially, need a lot of territory to stake out, (much like many animals do) and they define in a tank, based on fixed markers like certain rock structures, or maybe certain corals.
If they can't keep other fish out of their perceived territory, stress can cause the parasites that are always present, to break out in great numbers, causing death in many cases.
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