Drill the back glass on the center chamber. I've read the bottom is tempered, so you can't drill it. I've got a biocube 29 that I wish I'd drilled, but instead I went first with a custom 3/4 pvc siphon overflow and then a cpr cs-50 overflow. The pvc overflow wasn't reliable and while the cpr is more reliable, it required cutting the trim off the back of the tank, a big chunk out of the back of the hood and needs a very high water level in the tank. I suspect on a bc-12 there wouldn't be enough room for a cpr cs-50 anyhow.
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