My 90g has only a single over flow in one corner and I used both holes for overflows, one herbie and one straight pipe for emergency overflows. It's not that I'm worried about clogs it's that this way I can place my return line into the tank at the opposite corner of the over flow so it directs the water and any debris into the over flow instead of away from it making it more efficient. No need to drill anything, it doesn't take up any more space and also less plumbing for my return pipe as the return pump is directly under the opposite side of the tank from the overflow.
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