Ah, the plight of the beleaguered Anubias nana—deceptively robust, yet so commonly felled by subtle missteps in husbandry! Permit me to distill some key points and posit further possibilities.
First, as others have astutely noted, Anubias nana is almost archetypal in its aversion to direct or intense illumination. In nature, these plants thrive beneath the dappled shade of overhanging foliage or submerged in shallow, turbid streams. Ceaselessly bathing them in intense LED light can induce chlorosis—yellowing leaves—leading to a slow decline. If your tank is brightly lit, consider repositioning the plant beneath hardscape or driftwood, or moderating your photoperiod.
Equally insidious is the common error of substrate burial. The rhizome—the plant’s verdant spinal cord—must remain exposed to the water column. Burial restricts gas exchange, triggers rot, and, quite literally, asphyxiates the plant’s hopes for survival. Affix it delicately to stone or wood with thread or fishing line, allowing its roots to dangle free.
Water quality is your ally, but not the sole arbiter. Ensure gentle flow to discourage algae on leaves, avoid copper-laced medications (especially with invertebrates), and resist the temptation to over-fertilize.
In summation, let your Anubias bask in shadow and breathe freely, and it shall reward you with stoic, emerald beauty—a living anchor amidst aquatic tumult.