What causes a reef tank to crash?

Almost always instability or neglect, not bad luck. The big ones are alkalinity swings, salinity creeping up from missed top-offs, skipped testing and water changes, a dead animal or a melting anemone fouling the water, and heat or power failures. Adding too much livestock too fast overloads a young tank the same way.

What are RTN and STN?

Rapid Tissue Necrosis (RTN) and Slow Tissue Necrosis (STN) are when a stony coral, usually SPS, loses tissue and exposes bare white skeleton. RTN can strip an acropora colony in hours; STN creeps, often up from the base. Both are commonly triggered by parameter swings, pests, or nutrients bottoming out.

How do I stop RTN spreading?

Act fast: check and stabilise your parameters (especially alkalinity), and frag off the healthy tissue above the receding line to save the colony. Inspect for pests. Because it can move in hours on acropora, this is a same-day job - and a reason to keep SPS only once your tank is genuinely stable.

Why does alkalinity matter so much for stability?

Alkalinity is the parameter corals are most sensitive to, and it is consumed continuously by growing stony corals, so it drifts faster than anything else. A swing burns coral tissue and triggers browning and necrosis. Holding alkalinity steady is the single most important stability habit.

What should I do if my tank is crashing right now?

Test immediately, remove any dead animal or rotting matter, run fresh carbon, and do a measured water change with new saltwater matched to the tank. Stabilise rather than chase numbers, and get experienced help. The goal is to stop the cascade, not to fix every parameter at once.

Preventing a Reef Tank Crash

🐠 The short version: reef tanks rarely crash from bad luck — they crash from instability and neglect. Alkalinity swings, salinity creep from missed top-offs, skipped testing, a dead animal or a melting anemone, heat and power failures. Stony corals show it as RTN/STN (tissue dying back to white skeleton). The whole defence is a steady routine and catching problems early.

🔧 Stability is measurable — keep the numbers steady on the parameter chart and hold alkalinity with the dosing calculator.

What “a crash” actually is

A crash is a cascade: something destabilises the water, corals and fish stress, one death fouls the water further, and the whole system tips. It looks sudden but almost always has a traceable cause — which is good news, because that means it’s preventable.

The usual causes

Alkalinity swings — the number-one killer. Alk is consumed continuously by growing corals and is the parameter they’re most sensitive to. Let it drift or lurch and you get browning, recession and necrosis. See reef water chemistry.
Salinity creep. Evaporation concentrates salt; miss your top-offs and salinity climbs steadily. An auto top-off or a strict daily habit prevents it.
Skipped testing and water changes. You can’t defend stability you’re not measuring, and neglect lets nutrients and imbalance build.
A dead animal or a melting anemone. A large death — especially an anemone melting down — can foul a tank fast.
Heat and power. Summer heat or an outage can cook or de-oxygenate a tank; have a plan.
Adding too much, too fast. Overloading a young biological filter spikes ammonia.

RTN and STN: when SPS turn to skeleton

On stony corals, especially SPS, instability shows as tissue necrosis — the living tissue dies back and exposes bare white skeleton:

RTN (Rapid Tissue Necrosis) can strip an acropora colony in hours.
STN (Slow Tissue Necrosis) creeps, often up from the base — classic on birdsnest.

Common triggers: a parameter swing (alkalinity again), pests (AEFW, red bugs on acropora), or nutrients bottoming out. The response is fast: stabilise parameters, frag the healthy tissue above the receding line to save what you can, and check for pests. RTN’s speed is exactly why SPS belong only in a genuinely stable, mature tank (coral care ladder).

How to prevent a crash

Prevention is just stability made into habit:

Hold alkalinity steady — steady dosing sized to your tank’s uptake (the dosing calculator does the maths), not lurching corrections.
Automate salinity — an ATO, or never miss a top-off.
Small, regular water changes and regular testing — catch drift before it becomes a swing.
Don’t chase numbers. A steady value slightly off ideal beats a “corrected” number that swings. Adjust gently, within safe daily limits.
Go slow adding livestock, and have a heat/power plan (a battery air pump is cheap insurance).

If a crash is happening

Triage, don’t panic:

Test to see what moved.
Remove any dead animal or rotting matter.
Run fresh carbon to pull toxins/allelopathy.
Do a measured water change with new saltwater matched to the tank’s salinity and temperature.
Stabilise, don’t chase — bring things back gently, and get experienced help.

The goal is to stop the cascade, not to fix every parameter at once — a big fast “correction” is its own swing.

Sources & further reading

Coralkin — Reef tank water chemistry explained and the parameter chart — the stability targets.
Coralkin — Dosing a reef tank — holding alkalinity steady, the core of stability.
Coralkin — Water-chemistry questions.
Bulk Reef Supply and WetWebMedia — stability, RTN/STN and troubleshooting.