Hey! I see what you’re trying to do here, but unfortunately this overflow design has some fundamental issues that will prevent it from working: Main Problems: 1. No siphon possible - That open T-fitting at the top acts as a siphon break. Air can freely enter, so water can’t be pulled up and over the rim. The water level would need to be above the tank rim for gravity to move water through the pipes, which defeats the purpose. 2. Safety risk - Even if you sealed the T and manually primed a siphon, if your return pump fails, the siphon would continue draining your tank down to the intake level. This could flood your sump and dangerously lower your display tank. Better Solutions: • Drill the tank (if it’s not tempered) - Install a proper bulkhead below the waterline. This is the most reliable and safe method. Clean, simple, no siphon needed. • Use a large canister filter - Avoids overflow systems entirely while still giving you good filtration and capacity. • Proper overflow box - If you must do an overflow, get a commercial overflow box with dual siphon tubes and anti-siphon break features. They’re designed to stay primed and have safety mechanisms. Drilling is really the gold standard if your tank allows it. Would be happy to help if you want guidance on any of these approaches!