1) Get rid of the prefilter sponge (prone to clogging) and replace it with a Stockman standpipe. The Stockman will greatly reduce the gurgling noise that plagues single drain HOB overflow set ups.
Durso Standpipes are used to make your aquarium overflow (weir) nearly silent. We help you make your own or purchase them ready made!
www.dursostandpipes.com
2) In a single drain set up, NEVER use a valve to restrict flow in any way. Run it wide open. This will help reduce chances of anything getting caught in the restriction and causing a potential flood.
3) 3/4” inches would work well. If using the U-tube return pipe linked to below, be sure to drill a hole near the surface but just below the waterline, about 1/2 an inch, for a siphon break so if you kill or lose power to the return pump, you will back siphon less water back to sump, again avoiding a potential flood situation.
Here is a good thread on HOB overflows with a lot of tips to successfully run an HOB overflow. If planned and executed properly, an HOB overflow should serve you well.
I’m starting to consider adding a sump to my 45 cube reef because the maintenance on a canister is starting to get annoying. But the problem is I can’t drill the tank because it’s up and running. Are overflow boxes a reliable option?
www.reef2reef.com