I have a 55G aquarium that I am setting up for saltwater fish, probably clownfish. It includes a custom built-in overflow to the sump. The water level in the overflow is low. Is this to be expected or should there be a certain level of water in the overflow? If so how can I change this?
Just as I understand you right, you wonder how much water there should be in the left one on the picture?
I would say zero water, it depends on how high your pipe is from the sump. You build it like that so there will be enough big hole so you don´t overfill you aquarium. You will pump water up and it needs to have somewere to go else it will go over your edge and you have water everywere
It looks like you are missing the stand pipe for the drain. A common design out there is the durso standpipe. You can make one yourself from PVC pipe or buy one from their website. There are other designs out there but this one is supposed to be one of the quieter ones(and the one I'm going to try on my new reef tank).