The short answer
Brown aquarium leaves usually mean one of three things: the plant is melting after being moved, it has a nutrient deficiency, or the surface is coated in brown algae. The pattern tells you which — melt browns whole leaves at once, deficiencies brown the edges and older leaves, and algae forms a coating you can rub off.
Melt: new plants and crypts
If browning started soon after planting, it’s almost certainly melt — the plant shedding its old, air-grown (emersed) leaves as it converts to underwater growth. Crypts are notorious for it, sometimes dissolving to nothing before regrowing. Don’t uproot the plant: the roots and rhizome are alive and will push fresh leaves within a few weeks. Full detail in how to stop plants melting and why plants melt.
Deficiency: edges, tips and old leaves
Brown, crispy edges or brown blotches creeping in from the tips of older leaves point to a nutrient shortfall — often potassium or a broader lack of trace elements. The cure is a consistent all-in-one fertiliser, plus root tabs for root-feeders like swords and crypts. See our fertiliser picks and the fertiliser hub.
Algae: a brown coating
A soft brown dust on leaves and glass is diatoms, common in the first weeks of a new tank and usually fading on its own as the tank matures. Persistent brown or black tufts are different algae driven by imbalance — our brown algae and algae on plants answers cover both. When in doubt, tighten your light schedule and dosing so the plants outcompete the algae.