There is a white threaded fitting poking thru the back of the livewell, that is your overflow. Those drain to the big white thru hull fitting on the flat spot overhead from the drain plug…you probably never noticed unless you looked down there.
If you turn the ProAir fill and the aerators on the overflow cannot keep up with that much water being pumped into the livewell and it will overflow. Nothing wrong with your livewell system except great pumps.
You have to keep an eye on this and just use the ProAir fill and pump outs as just that, fill and pump outs. Use the aerators and recirc pumps once the livewell is full that will help you.
I hope this helps…you definitely do not want those valves turned open unless you are cleaning the spray bars, that is all they are for.
Judd Lasiter