Drain it and drill another drain hole for emergency which sits higher than the current one.
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You should make an overflow box drill another hole for a 3/4 bulkhead next to the current drain and make the 3/4" an emergency drain and add a ball or gate valve to the half inch drain and run it at a full siphon. it would be super easy to attach an acrylic internal overflow box being that they would both be acrylic (weldon 4 solvent). that how I would do it, its called a herbie drain and it would give you a completely silent drain. hope this helpsLooks like I should have done a 3/4 overflow. At least I know for next time. The overflow is back to the original boring open ended pvc, and lll get a glass holes overflow skimmer to keep snail's out.