another pony fan!! When i inquired on some pony info Yvette put me onto this post.
https://seahorsewhisperer.wordpress...ls-why-buying-from-a-breeder-is-so-important/
pretty good read on things to avoid.
I went back through a few of the photos and its really hard to see but are your 2 overflows not level with each other? It is "possible" but not "probable" that the 1 is draining because its lower and is lowering your oveflow height. Maybe it was just the way you took the photo but the one photro looks like they are about a 1/4" out which may explain the differences in flow. Try swapping the gate valve to the other overflow if thats easily possible and throttle it. If the other one starts to drain more than maybe that proves it? In that case you could likely leave the other one valved?
Could be way off on this one....
Good luck! everything is looking really nice.