🏥 Free tool · triage, not a diagnosis
Is my fish, coral or invert okay?
Something looks off and you want a calm, honest read. Pick what you're worried about, tick what you're seeing, and we'll suggest possible causes and safe first steps — always leading with the water. This never diagnoses and never gives a medication dose; when something needs a vet or experienced eyes, it says so.
1 · What are you worried about?
2 · Tick what you're seeing
🐟 fish — start here
- Test the water first — ammonia and nitrite should both read 0, and nitrate should be low. A hidden water problem mimics almost every disease.
- Check salinity and temperature are in range and STABLE (≈35 ppt / 1.026, 76–82°F). A swing stresses fish and invites disease.
- Boost oxygen and flow (surface agitation), stop overfeeding, and remove any uneaten food or a dead animal.
- Never medicate the display reef — copper and most fish meds kill corals and inverts. Treat a sick fish in a separate quarantine/hospital tank.
- Fine gold/rust "dusting" + very rapid breathing + fish dying within a day or two (possible marine velvet — moves fast, treat as an emergency)
- Several fish sick or dying at once
- Not eating for several days with visible weight loss ("pinched" belly)
- Open ulcers, sores, or a fuzzy grey film spreading over the body
- Rapid, labored breathing or gasping at the surface that a water change and aeration don't fix
Common signs
White spots like grains of salt/sugar
Possible causes: Marine ich (Cryptocaryon) — the most common cause of raised, salt-grain-sized white spots that come and go; Sometimes stress or a minor irritation.
Safe first step: Don't medicate the reef. Keep water pristine and stable, feed well, and improve oxygen. The reliable cure is treating the fish in a quarantine/hospital tank under a proper protocol — and letting the display run fallow (fishless) so the parasite dies off. Read our ich & velvet guide before acting.
A few spots that vanish overnight and never return may just be stress. Spots that multiply, or come with scratching and fast breathing, are the concern.
Fine gold/rust "dusting" + very fast breathing
Possible causes: Marine velvet (Amyloodinium) — a fine gold/grey dusting with rapid gill breathing, and it can kill a whole tank in days.
Safe first step: Treat this as an EMERGENCY. It attacks the gills, so add oxygen/surface agitation immediately and move affected fish to a quarantine tank for proper treatment — this is not a wait-and-see. Get an aquatic vet or an experienced reefer involved fast. Never dose the display.
Velvet is far faster and deadlier than ich — rapid breathing + a fine dusting + sudden deaths is the pattern to act on now.
Scratching / "flashing" against rocks or sand
Possible causes: External parasites (ich/velvet/flukes) in the early stage; Gill or skin irritation from poor water or high ammonia.
Safe first step: Test the water first (ammonia/nitrite 0), then watch closely for spots or a dusting. Flashing is often the earliest warning of a parasite — get a quarantine plan ready before it spreads.
Not eating
Possible causes: Stress from a recent move or a new tank; Water-quality problem; Wrong food or a shy/finicky species; The start of an illness when combined with other signs.
Safe first step: Check the water and give a new arrival quiet time and cover. Offer a variety of foods (frozen mysis often tempts a picky fish). A few days off food is common; a week-plus with weight loss is the worry.
Rapid or labored breathing / gasping at the surface
Possible causes: Low oxygen or high temperature; Ammonia spike; Gill parasites (velvet/flukes).
Safe first step: Add surface agitation/aeration and do a water test now. If temperature is high, cool the tank. If breathing stays fast with a fine dusting on the body, treat as possible velvet (see above) and act fast.
Clamped fins, dark colour, hiding, or "stress bars"
Possible causes: Stress — new tank, aggression from a tankmate, or a water problem; The general early sign of feeling unwell.
Safe first step: Check water, watch for bullying, and provide caves/cover. Persistent clamping with other signs (spots, not eating) means look harder and prepare quarantine.
Cloudy eyes, or a fuzzy grey/white film or patches
Possible causes: A bacterial or fungal infection, often secondary to poor water or an injury; An excess slime-coat response to an irritant.
Safe first step: Get the water pristine and stable first — many mild cases clear once conditions improve. If it spreads or the fish declines, quarantine and get an aquatic vet or experienced reefer's advice. No home dosing of the reef.
Open sores, red ulcers, or ragged/rotting fins
Possible causes: A bacterial infection — often serious and can move fast; An untreated injury in poor water.
Safe first step: Pristine water immediately, quarantine the fish, and get an aquatic vet or experienced reefer involved — bacterial ulcers usually need proper treatment in a hospital tank, not a display additive.
Stringy white feces / a pinched, hollow belly
Possible causes: An internal parasite or gut infection (common in wild-caught fish, e.g. some tangs); Not enough of the right food.
Safe first step: Offer varied, enriched foods (some keepers use medicated/soaked foods under guidance). Persistent white stringy feces with weight loss warrants a quarantine plan and advice from an aquatic vet or experienced reefer.
Several fish sick or dying at once
Possible causes: Marine velvet or another fast disease; A water-quality crash, an ammonia spike, or low oxygen; A toxin (palytoxin from zoas, a chemical, an overdose).
Safe first step: This is an emergency. Add aeration, test the water, and do a large water change with matched-parameter saltwater. Move survivors to clean, oxygenated water if you can, and get experienced help immediately.
🐠 coral — start here
- Test alkalinity, calcium and magnesium — a swing (especially alkalinity) is the #1 cause of unhappy corals. Stability matters more than the exact number.
- Check temperature and light: a heat spike or a sudden lighting change (new light, moved coral) bleaches corals fast.
- Check flow and placement — too much or too little flow, or a spot that's too bright/dim, makes a coral sulk.
- Check your nutrients: nitrate and phosphate should be low but NOT zero — bottomed-out nutrients pale and starve corals.
- Tissue visibly receding, peeling, or "melting" off the skeleton (SPS: STN/RTN — can spread in hours)
- A brown, jelly-like slime eating an LPS coral overnight (brown jelly — isolate it fast)
- Rapid bleaching (going white) across multiple corals — check for a heat spike or parameter crash first
- A coral that has died and is fouling the water — remove it before it crashes the tank
Common signs
Not opening / polyps staying retracted
Possible causes: Recently added or moved (settling in); Flow too strong or too weak; A parameter swing or a pest bothering it; Light too intense.
Safe first step: Give a new/moved coral a week to settle. Otherwise check alkalinity stability, flow and placement, and look closely (day and night) for pests. One sulking coral in an otherwise happy tank is usually placement or flow.
Turning pale or white (bleaching)
Possible causes: A temperature spike (the classic trigger); A sudden lighting change (new light, or moved up into stronger light); A parameter crash — especially alkalinity.
Safe first step: Check temperature and recent light/parameter changes first. Stabilise conditions and, if you just changed lights, reduce intensity/photoperiod. A coral that keeps some colour can often recover if you fix the cause quickly.
Tissue receding, peeling, or "melting" off the skeleton
Possible causes: Alkalinity/parameter swing (the top cause for SPS); Infection or a pest; A sting from a neighbour.
Safe first step: Act fast, especially for SPS — check and STABILISE alkalinity, calcium and magnesium, and look for pests or aggression. Many keepers frag off the healthy tissue to save it. Isolate a coral that's actively losing tissue.
RTN (rapid tissue necrosis) can strip an SPS colony in hours — this is the coral equivalent of an emergency.
Brown jelly-like slime eating an LPS coral
Possible causes: Brown jelly "disease" — an infection that spreads fast over stressed LPS (hammer, torch, frogspawn), often after damage or a parameter swing.
Safe first step: Isolate the coral immediately, gently baste/dip off the jelly in clean tank water, and frag away affected heads to save the healthy ones. It can consume a colony overnight, so move quickly.
Algae, or visible pests, overgrowing a coral
Possible causes: Nutrient imbalance feeding algae; Coral pests (aiptasia, flatworms, montipora-eating nudibranchs, etc.).
Safe first step: Identify before you treat: get flow and nutrients balanced for algae, and dip/quarantine new corals to keep pests out. Match the fix to the specific pest — our algae & pests guide walks through the common ones.
🦐 invert (shrimp, snail, crab, anemone…) — start here
- Confirm there is NO copper in the tank — inverts have zero tolerance for it, and copper-based fish meds are the classic invert killer.
- Test the water and check salinity/temperature are stable — inverts are very sensitive to swings and to un-cycled tanks.
- Remember a shed exoskeleton is a MOULT, not a death — shrimp and crabs leave a hollow "ghost" shell and hide while their new shell hardens.
- Make sure new arrivals were DRIP-acclimated slowly — most invert losses trace to a too-fast salinity/temperature change on arrival.
- An anemone deflating, mouth gaping, or expelling its insides, and not recovering (test water and be ready to remove it before it fouls the tank)
- An invert that died shortly after any medication went in the tank (suspect copper — do a water change and run carbon)
- A shrimp or crab stuck part-way out of a moult for more than a day
- Multiple inverts dying at once (check copper, salinity, and oxygen)
Common signs
An invert died after medication or a new additive
Possible causes: Copper or another invert-toxic medication in the tank — inverts have zero tolerance for copper.
Safe first step: Do a water change, run fresh carbon, and test for copper. Never dose copper (or most fish meds) in a tank with inverts or corals — treat fish in a separate quarantine tank instead.
A hollow "dead shrimp/crab" — or one stuck in its moult
Possible causes: Usually a MOULT — shrimp and crabs shed a hollow exoskeleton and hide while the new one hardens (this is normal and healthy); A genuinely stuck/failed moult can signal a nutrition or iodine issue.
Safe first step: Look for the animal hiding nearby — a clear, hollow "ghost" shell means a healthy moult, not a death. Leave the shed to be eaten (it recycles minerals). If an animal is truly stuck mid-moult for over a day, check parameters and diet.
A snail/invert not moving, or upside-down
Possible causes: Too-fast acclimation (salinity/temperature shock); A water-quality or copper problem; Some snails simply right themselves slowly — give it time.
Safe first step: Check salinity, temperature and copper, and gently turn a stranded snail upright near the rock. New inverts must be DRIP-acclimated slowly — most losses trace to a rushed acclimation.
An anemone deflated, gaping, or wandering constantly
Possible causes: A water-quality or stability problem; Too little light or the wrong flow; A tank that isn't mature enough for an anemone.
Safe first step: Test the water and check light/flow and stability. A mouth that gapes open and won't close, or "insides" coming out, is a failing anemone — be ready to remove it before it fouls the tank. Screen powerheads so a wanderer isn't shredded.
Host anemones need a MATURE, stable tank — adding one too early is the most common reason they fail.
Quick answers
Does this tool diagnose my fish or coral?
No — and it never prescribes a medication or a dose. It suggests possible causes and safe first steps for the signs you tick, always leads with the water, and shows you when to get an aquatic vet or an experienced reefer involved. It can't see your tank; a vet or an experienced keeper can.
My saltwater fish has white spots — is it ich?
Raised, salt-grain-sized white spots that come and go are most often marine ich (Cryptocaryon). Don't medicate the display reef — copper and most fish meds kill corals and inverts. Keep the water pristine and stable, and treat the fish in a separate quarantine tank under a proper protocol. See our marine ich & velvet guide before acting.
What's the difference between marine ich and velvet?
Ich shows as distinct salt-grain white spots and progresses over days. Velvet (Amyloodinium) is a fine gold/grey "dusting" with very rapid breathing and can kill a whole tank in a day or two — it's a true emergency. If you see fast breathing plus a fine dust plus sudden deaths, act immediately: add oxygen and get affected fish into quarantine treatment.
Why does my coral keep closing up / turning pale?
A coral that won't open is usually telling you about flow, light, placement, or an alkalinity swing — check those first. Going pale or white (bleaching) is most often a temperature spike or a sudden lighting change. Stabilise the cause and most corals that keep some colour recover.
My shrimp/crab looks dead — is it?
Probably not — a clear, hollow "ghost" shell means a healthy moult, not a death. Shrimp and crabs shed their exoskeleton and hide while the new one hardens. Look for the animal nearby, and leave the shed to be eaten (it recycles minerals).
Go deeper: Marine ich & velvet ·How to quarantine ·Parameter targets ·Check your test results