Finding pin holes in a closed loop system

[my2rotties]

I cant get photos at the moment. The display is above the sump and fuge. The water goes from the sump, up into the pump, to the tank, back out through the overflows and into the sump again. Also the skimmer in the sump is not longer there. I guess the other skimmer blocks the view of the plumbing going upstairs to the display...

My husband built it and I just maintain it. He is not available this evening due to hockey so I'm on my own and don't know much about the guts of the system.