Hang on back overflow box


Reefing newb
i have a 55 gallon saltwater fish only right now. The tank is not drilled
i am looking at a hang on the back overflow why are PVC pipes used? could you use lengths of vinyl hose?
how big of a tank for the sump for a 55 gallon. i have heard half the amount. but no reference or reason as to why.
I am looking at an auto shut off valve for the return pump as a solution to loss of siphon, thoughts or other solutions?
I just got my sump set up for my 55 Gallon reef tank 3 weeks ago. I used Trigger's Crystal 30 inch sump (holds about 20 gallons), a Marine Depot OF-800 overflow box and a Current Loop eFlux DC pump (1050GPH). All is working well after some initial setup.
I got PVC pipe from our local hardware store for the return from tank to sump (has 2 elbows) and 1 inch flex tube for the return pump to the overflow U return directional return pipe ($10 off of Amazon).

I was also concerned with the siphon loss and having the return pump overflow my tank to my floor. Took care of that with AutoAqua's smart level security ($60 from Marine Depot). This eye (much like an ATO eye) sets about an inch below my water level in my sump (1st return area). If the siphon breaks (which it has not, even after a water change) and the return pump starts pumping my sump water into the display tank, the AutoAqua security will shut the power off to the DC return pump well before my display tank is completely full.
If, on the other hand, the pump fails and too much water starts coming into your sump, just make sure your overflow box is at a good level to not let too much get in AND make sure your overflow return is not too low to get back flow there.
When I did my two tests everything worked as planned - 1) I took out the siphon tube, breaking the siphon - the overflow quit working and the pump kept pumping, but the AutoAqua turned it off in plenty of time (when the sump got down an inch the pump shut off). 2) With the siphon back, I turned off the return pump, water came in a bit from the overflow to the sump and a bit came back through the overflow U pipe, but only raised my sump water about 2 inches - no where near overflowing. So I think I'm safe here.

So... to answer your question about sump size. If you take the right precautions it's really a matter of space and personal preference. The 25-50% range of your main tank is normal (mine is around 35%). IMO, if you set it up correctly - you will never regret going with a sump. So much easier to maintain water quality and all of the unsightly equipment is out of your display tank with more space for your critters.