It's not an overflow, it's a sealed closed canister filter loop, no sump in other words.

So the water is taken in down on the bottom, cycled through the Canister, then sent to heat and CO2, then back to the bottom.

I use heavy duty Sch 80 Bulk heads with seals on both sides and E6000 glue as well for long term seals. A good wipe with silicone lubricant for rubber gasket seals helps as well.

Regards,
Tom Barr