A new clownfish goes into the tank. Two days later it is covered in something white and breathing hard, and by day three it is gone. Everyone tells you it was ich. Often it was not, and the difference determines whether the rest of your fish survive the week.
Marine ich (Cryptocaryon irritans)
Ich shows as distinct white spots, roughly the size of a grain of salt, scattered on the body and fins. You can count them. The fish typically flashes and scratches against rock, and the spots come and go in cycles as the parasite drops off to reproduce and returns in larger numbers.
Ich is serious, but it is comparatively slow. You usually have days to act, and a fish that is eating and not gasping has a real chance.
Brooklynella (Brooklynella hostilis)
Brooklynella looks nothing like discrete spots. It presents as sloughing, milky-white excess mucus peeling off the body in sheets or patches, heavy and rapid gill breathing, and often a fish that stops eating almost immediately. It attacks the gills first, which is why respiratory distress leads the symptom list.
The critical difference is speed. Brooklynella can kill a fish in 24 to 48 hours. It is called clownfish disease because wild-caught and poorly-handled clowns are its classic victims, and it hits hardest right after shipping stress.
The one-look test
Ask yourself: can I count the spots?
- Countable, salt-grain spots + scratching, over several days → think ich.
- Sheets of white slime + heavy gill breathing + a fast crash → think brooklynella, and act today.
Two more that get confused with both: velvet (Amyloodinium) gives a fine, gold-dusted sheen and kills even faster than brooklynella, and uronema shows as red, ulcerated sores rather than spots, usually in chromis and anthias. Different organisms, different treatments, and a wrong guess costs the fish.
Why this matters more than the treatment details
The treatments genuinely diverge. Ich protocols center on copper or tank-transfer over a period of weeks. Brooklynella responds to formalin dips, and it responds to them urgently. Treat brooklynella like ich and the fish dies while you are being patient. Treat with the wrong medication and you have added toxicity to an already-failing animal.
Two things are true regardless of the diagnosis, and they are the ones worth internalizing:
Almost none of this belongs in your display tank. Copper is a disaster for invertebrates, medications wreck biofiltration, and a reef tank cannot be sterilized once a parasite is established. Treatment happens in a separate bare-bottom hospital tank. If you keep fish, having a plan for a hospital tank before you need one is the difference between an inconvenience and a wipeout.
Quarantine every new fish. Two to four weeks in observation, on its own, before it goes near the display. Every serious outbreak arrives with a new arrival, and quarantine is the only intervention that consistently prevents them. It is also free.
Get the ID right, fast
When a fish is in trouble, you have hours, not evenings for forum research. Our Fish Disease & Parasite ID Guide is a 30-page reference built for that moment — ich, brooklynella, velvet, uronema, flukes, bacterial infections and more, with photos, the distinguishing symptoms, treatment approaches and quarantine protocols, laid out so you can find the answer in a minute.
It is an instant PDF download, so it is on your phone before the next problem starts. It is also in the Pest & Disease Bundle and the Complete Reef Care Library.
This guide is educational reference material, not veterinary advice.