STN (Slow Tissue Necrosis) is the chronic form of coral tissue loss, taking days to weeks to advance. Often triggered by chronic low light, low alk, or trace-element imbalance.
Practical use in aquariums
Diagnosis: STN advances at the base or edge of the coral; healthy tissue still extends polyps. Treatment: ICP-MS test, increase alk to 8-9 dKH if low, check PAR + replace bulbs/LEDs older than 18 months. Frag the healthy tissue away from the necrotic edge as a backup if advance does not slow within 2 weeks.
How STN fits the bigger picture
Understanding STN matters because it's connected to broader husbandry decisions: disease prevention is cheaper than treatment - quarantine + dipping protocols paid back the first time you avoid a tank-wide outbreak.
Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.