I think I've posted on similar scenarios about a million times already, but I'll do it again because why not. You can use a gate valve on the primary siphon line to raise the water level in the overflow box and quiet the drain to some extent BUT, seeing everyone for some strange reason thinks their tank needs three 1" or larger drains regardless of tank size and return pump flow, it's always going to be noisy no matter how much you try to "tune it" with a durso pipe, silencers or adjusting gate valves. I've seen probably a few hundred tanks in person and WAY too many people do exactly this, yet swear up and down how quiet their tanks are...even if it's only because the skimmer or wavemakers are so loud that they at least distract away from the constant gurgling noises, or the person has a dedicated "fish room" or garage that houses the sump and moves a lot of that noise some place else. I prefer to address the issue and not just mask it. The way you build a quiet drain system is by trying to fill the primary siphon drain pipe as much as possible, while displacing as many rising air bubbles coming back up the line from the sump as possible and then have your secondary, typically durso drain pick up the slack, which hopefully should be barely any remaining water flow. The problem is, if your return pump is only sending 500gph of flow to the display and the 1" primary full-siphon drain that handles around 1,700 gph fully open, there's going to be a vertical air gap along the entire length of that drain to "waterfall" directly into the sump. You can use a gate valve to reduce the flow BUT, the volume of the drain STILL won't be filled to prevent a large air pocket from forming the the pipe and you'll STILL have "waterfall" noise. If i'm correct and you're building a drain system for a 50g tank, you want a 1/2" - 3/4" primary siphon drain. A 1/2" will pair well with a return pump that will flow up to 450gph into the display (you have to calculate head height for this). A 3/4" will work good with a return pump that flows up to 800 gph. You'll still have to use a gate valve, but instead of having to close it 99% of the way with a 1" drain, you can have the 1/2" line's open halfway or more, which helps purge air out of the line past the gate valve and back into the sump. Between the primary and secondary lines, you'll have plenty of safety margin just in case something clogs and for the emergency drain, make it as large as you want because it should only operate as the name implies and hopefully be noisy enough to get your attention, which will especially be the case if your other drains are dead-silent. I personally ran a 100g mixed reef with 450gph return pump flow going to the tank. My drain sizes were 1/2", 3/4" and 1". It was nearly as quiet as a siphon overflow box setup and you'd have to stand a foot away from the drain and listen carefully to be able to notice it. I "stress tested" it numerous ways including creating clogging scenarios and simulated power outages and never had a situation where the secondary drain couldn't carry the entirety of the flow, especially when its little siphon line became submerged.
