Ya, being serious for just a moment, yes, you're right. A tang needs a 6' tank. Except what if that tank is full of rock. Then an empty 4' tank is better. Right? Maybe, maybe not.
If you can maintain water quality, fish don't get sick from stress, don't kill each other fighting, and there's room for all of them to be fully submerged, then you're good.
Unless you're not. It depends.
Regardless of the state of this particular tank's stocking, I don't think it has any bearing on the fish dying. I'd have to double check with my brain tho, it may disagree.
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Brad
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