Noticed a small white worm bloom crawling the glass near the substrate line. Not harmful on their own, but a symptom — anaerobic pockets forming under the aquasoil where mulm had built up, plus a stretch of slight overfeeding while I was dialing in the CO₂.
Cut feeding back, ran a gravel vac pass through the low-flow corners, and the population dropped off within a week. Note to future self: overfeeding shows up in the substrate before it shows up anywhere else.