Plumbing Question

So I now have my 120g tank set up (with 30g sump), I have an established intake and output pump. But I am wondering about how other reefers have set up an emergency return or intake shut off (whichever), for whenever there is a power out to keep water cycling.

Below is my paint example of what I currently have going. (its very hard with my tank stand and camera to actually see anything .-. So I am sorry about the poor example here) On the left is my intake, the silver snake in the back of the tank is a vinyl tube that connects to the water pump in the sump, and the purple is my skimmer and ofc my two powerheads with arrows pointing out flow direction. The red circle is where I want to know if I could put a return line of pvc and get a reverse suction going or something? (I know gravity would not be my friend here just trying to get an idea)

So far the intake and pump work great! I have them balanced perfectly it's just I worry about if my power ever goes out, am I going to have a flood or will the water stop cycling. So some insight or a point to a thread in the right direction would be super awesome and greatly appreciated! Again sorry for poor example, the only good lighting in this room is for the fishies so other than inside the tank, cant see very well the outside lol

 

Read this article on sumps.
What is a sump? | Melev's Reef

You should have an overflow box, either a hang on back or an internal with a drilled tank to get wate rfrom the display down to the sump and a pump to get it back up from the sump to the display. The overflow works by siphon and should be sized larger than the return pump so there is no danger of a flood. There really is no balancing, the overflow will be 100% unrestricted and the return pump will balance itself out with the overflow if sized properly or at most the return can have a ball valve on the discharge side to throttle flows a little but you never restrict the overflow.

Photos and a better description would really help.

 

If your set up is as drawn then you will have a problem with a power outage.

Here will be the sequence of events:

- Power goes out, pumps stop
- The grey drain line on the left will siphon the water out of the tank down to the level of the intake in the tank.
- The siphon in the line will now break.
- Power comes back on.
- Pump starts filling tank.
- Since your drain line goes over the top of the tank and its siphon is broken, it has no way to get it back again.
- Pump keeps pumping, nothing leaves the tank, tank overflows onto floor.

Get a hang-on-back overflow unit and all will be well. No need to do anything else.