SPS Troubleshooting: STN, RTN, and Browning

When SPS Corals Show Signs of Trouble

SPS keeping is not all growth tips and fluorescing colonies. Understanding how to identify and respond to common problems can save entire colonies.

STN: Slow Tissue Necrosis

What it looks like: Tissue gradually recedes from the base upward, exposing white skeleton. Progress is slow — millimetres per day.

Common causes: Poor flow at the base, alkalinity instability, encroaching neighbours stinging the base, or pests feeding at the base.

Response:

1. Increase flow around the affected area
2. Test alk at the same time daily for a week to check for swings
3. Inspect the base with a magnifying glass for pests
4. If progressing, frag above the recession line — STN rarely reverses once established

RTN: Rapid Tissue Necrosis

What it looks like: Tissue sloughs off in hours, leaving white skeleton behind.

Common causes: Sudden parameter shifts (especially alkalinity), temperature spikes above 28°C, bacterial infection, or coral stinging. Sometimes no identifiable trigger.

Emergency response:

1. Frag immediately — cut well above the recession line (2cm minimum above dying tissue)
2. Dip the frag in iodine-based dip (diluted Lugol's solution)
3. Place in a high-flow area or separate container with clean water
4. Check parameters — especially alk, temperature, and salinity

Browning Out

What it looks like: Once-vivid SPS turns brown or muddy.

Common causes: Too much light too fast, high nutrients (nitrate/phosphate), insufficient light, or normal new-frag stress.

Response: If nutrient-driven, improve export and bring nitrate under 10ppm and phosphate under 0.08ppm gradually. If light-driven, adjust intensity or position. New frags often colour back up after 4–8 weeks of stable conditions.

Pale Tips / Burnt Tips

Bright white tips are usually normal growth — new skeleton before zooxanthellae colonise it. Burned or necrotic tips suggest too much light or an alk spike.

Prevention Is the Best Medicine

Most SPS problems trace back to unstable alkalinity, poor flow, moving corals around too often, or not dipping new additions. Stable and boring is exactly what SPS want.

Has anyone dealt with STN or RTN? What helped — or what would have helped in hindsight? Share below.