Ugh, leak test failed today. I was most concerned about the leak occurring at one of the return bulkheads or the bulkhead between the overflow box and the tank. Oddly enough, however, it occurred on one of the bulkheads for the drain connections on the overflow box (closest one in the image below). I'm not sure what I am going to do yet. Of course it occurred on the part of my plumbing where I couldn't fit a union to ease disassembly and reuse!
Here's a picture of the sump for those that are interested.
I also learned a few things today. First, keep your returns pointed up when filling, not down. I didn't have the return pump plugged into the controller yet and the returns started a siphon down to the sump because they were pointed down below the drain level. Luckily, I caught it with the shutoff valves less than half an inch from the top of the sump.
Also, it looks like my pump has more than enough power to overwhelm the primary drain, so I'll need to really ramp it down when I get everything figured out.