Update. Been battling this for a while, and can't seem to get to the bottom of the nitrate. I drastically cut back on feeding, and it seems to have unintentionally thinned out my ghost shrimp, but I haven't found any dead ones. Fish are still acting normal, no signs of distress, healthy appetites, and still very active. BNs are doing the courtship thing - or at least that's what I've been told - but thankfully no breeding at this time.
Just did a massive water change, pulled out the marimo balls and squeezed them out in a bucket of tank water, and got a ton of stuff out of them. (They were relatively clean last big WC.) Also shook out my filter media in the dump bucket, but didn't replace any of it to avoid an ammonia spike on top of the nitrate issue. Also removed the ferns because they weren't doing so great - the moss had really grown out and was entangling and shading them. Pulled some more moss out too, as it's growing rather quickly and was getting too close to the filter intake.
I also moved decorations and vacuumed up all the debris I could find, refilled the tank, then vacuumed again.
Tested last night. Zero nitrite, negligible ammonia (between 0 and .25), but nitrates are still up around 40-80 (the colors on the card are very close, and the test vial is bright candy red). Plan on siphoning out more water tonight after work, but am wondering if the mopani leaching tannins can affect nitrate. I soaked the wood for roughly a year before using it, but it still leaches out some tannins. The wall behind the tank is dark avocado green, so the discoloration isn't noticeable until I siphon water into a white bucket. However I do notice how much brighter my tank looks when the tannins are substantially diluted due to a water change.
Could tannins contribute to the nitrate reading?