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It is very risky to rely on a siphon as an overflow because if the siphon breaks or the pump breaks or the power goes out it will overflow. Do you have any way to drill your tank? You can do it yourself really easily. You could drill it toward the bottom on the back wall and have a pipe go up to the top or drill it at the top and get an actual overflow box. What size is your tank? The 1/2 inch pvc cannot handle a lot of flow. Attached is a series of tests I found online that show flow rate through different size pvc pipes.