We ran fresh water through the system while we were tweaking the plumbing. Where the water came down from the overflow, we of course had some turbulence. Last night as we were draining the new fuge, we saw what looked like skimmate on the walls right at the water line. It was kind of a golden brown that felt slimy when wet but got crusty after it dried. It seemed to be attracted to the silicone on the bubble trap especially.
We hadn't noticed anything like that even though we had filled/drained the tank a couple of times during the last week. It made no sense and we got worried that maybe we had an oil leak. Turns out the two pumps are mag drives so that eliminates oil spill, but we still don't know what it could be. We got to thinking that the water always looked hazy when we looked through it long ways, even with multiple draining/refills. I had added a little bleach to it during the week to keep it from getting slimy, but this last filling didn't have any.
I think the plan is to run the fresh system with carbon and see if the gunk goes away. Hopefully by next weekend we'll have this taken care of.