Drilled Tank Question

[BT05]

Not sure if I'm reading this correctly but are you saying that the additional hole is drilled in the bottom outside of the overflow? Or do you have 2 holes drilled inside the bottom of overflow box; 1 for drainage, 1 for return?

In either case, you should have your main return line plumbing go above the water level, tee it off, then place the outlets down about an inch or two below the water line. If you just leave it as is and return water directly without *any* plumbing above the bulkhead (bottom of the glass), the entire tank will drain in a power outage. I wouldn't rely on a check valve regardless of the setup.

You can drill a hole in the return line but it needs to be fairly large in order to be effective and it can also get clogged with algae, snails, etc. once the siphon starts. One way to guard against power outages is to pick an appropriately sized sump that can hold the excess water siphoned from the tank when power goes out. This means that you keep the water line of the sump only halfway when the tank is in use. For your tank, maybe a 50+ gallon sump would do depending on how far down the return outlets go below the water level. Gives you a ~25 gallon room for error.