Aquatic plants are poikilothermic: their metabolism is directly regulated by water temperature. While tropical fish often prefer temperatures of 28-30°C, almost all aquarium plants (often of sub-tropical origin) thrive best between 21 and 24°C. Above 28°C, cellular respiration accelerates faster than photosynthesis: the plant literally burns more energy than it can produce, starving itself. A sudden temperature drop (e.g., during a winter water change) instead causes the collapse of lymphatic vessels.

In prolonged excessive heat (summer): stems become thin and spindly, the distance between internodes elongates drastically (etiolation), and basal leaves detach. The plant stops forming healthy new shoots. In sudden cold shock: leaves wilt instantly, becoming limp, translucent, and gelatinous within 24 hours, falling off the main stem.

Keeping the heater set too high (typical in Discus tanks where the temperature is 29-30°C). Summer heatwaves without the aid of cooling fans or chillers. Performing large water changes (e.g., 50%) using freezing tap water without temp-matching it first.

In summer or Discus tanks, the only real treatment is physically lowering the water temperature. Installing tangential fans over the water surface can drop the temperature by 2-4°C via evaporative cooling. In case of cold water shock, remove liquefied leaves to avoid ammonia spikes and patiently wait for regrowth from healthy lower nodes.

Keep the tank at 22-24°C for a lush planted tank (classic tropical fish like Neons or Rasboras live perfectly well at these temps). During winter water changes, always mix hot water with cold to exactly match the tank's temperature before pouring it in.