My first thought was high nitrates in the source water, then a bad test kit, but both have been ruled out. Clearly there's some unseen evil in the tank making it a nitrate factory. When all else fails, it's time for a tear down.
Substrate is black Petco brand aquarium sand. I did vac some debris last night and hit the areas with no plants. Also pulled the filter and cleaned it as best I could, and got a bunch of crud out of the pump housing. Changed filter media, took out about 8 gallons, and the nitrate dropped to around 20.
Nitrate climbed back up, but I tested after feeding. Going to do a water change tomorrow (today) after work. Interestingly, the ammonia stayed under .25 after messing with the filter. Zero nitrite, as usual. Fish still highly active and eating heartily, and Archer and Lana are chasing each other all over the tank. I don't know anything about breeding BNs, so I'm wondering if changing out so much water is triggering something.
It had dropped to 20, then was between 20 and 40 when I tested last night. But I had fed frozen bloodworms and a few shrimp pellets within an hour of testing.