Hi Luke,
Sounds like you have something I currently have on my 75 gallon, i have a hang on overflow going down to my homemade sump.
Now on my return pipe, I put a small hole near the top ( I had to measure when the pump was off, how much and where I wanted the water to stop returning to the sump)
However, After coming home to a water room (lol) one day, I decided to purchase a U.P.S. unit.
Below is the link of the current one I'm using for my aquarium with the overflow.
I hope this helps in some way!!
Here is something I use on my Overflow Tank
https://www.staples.com/APC-Back-UPS-XS-1500VA-10-Outlet-Power-Saving-UPS/product_797220