Treating Black Ich, black spot diese in a reef tank

by Joshua Karch

Black "ich" (also known as Black Spot Disease or Tang Disease) is a parasitic infestation that primarily affects marine fish, especially Tangs (Surgeonfish).

It's important to know that Black Ich is NOT the same as Marine Ich (White Spot Disease, Cryptocaryon irritans).

🔍 What is Black Ich?

  • Cause: Black Ich is caused by a tiny parasitic flatworm, typically a species of Turbellaria (most commonly Paravortex).
  • Appearance: The main symptom is tiny, raised black dots, about the size of a grain of salt, on the fish's body and fins. These black spots are the fish's own immune reaction (melanin pigmentation) to the flatworm burrowing into its skin.
  • Behavior: Infected fish may "flash" or rub themselves against rocks or substrate to try and dislodge the parasites.
  • Lifecycle: The adult flatworms eventually drop off the host fish into the substrate (sand/gravel), where they encyst and reproduce, releasing new juvenile worms to find new hosts. They can survive in the substrate without a fish host for several months.
  • Danger: While generally less immediately fatal than Marine Ich or Marine Velvet, a heavy or prolonged infestation can cause stress, lethargy, loss of appetite, and open the door for secondary bacterial infections.

💊 How to Treat Black Ich in a Reef Tank

The most effective and common treatments for Black Ich in a reef environment involve using medications that specifically target flatworms.

1. The Preferred Reef-Safe Treatment: Praziquantel

The medication Praziquantel (often sold under brand names like Prazipro) is the preferred treatment as it is highly effective against parasitic flatworms and is generally considered reef-safe (safe for corals and most invertebrates).

  • Treatment Method: Praziquantel can be dosed directly into the display tank.
    • Follow the manufacturer's instructions for dosing amount and duration.
    • Typically, it requires a single dose, possibly with a second dose 5-7 days later.
  • During Treatment:
    • Remove activated carbon and turn off your protein skimmer (or remove the collection cup) for the duration of the treatment, as these will remove the medication.
    • Since the medication can be light-sensitive, some hobbyists suggest dimming or reducing the duration of tank lights for the first 24 hours.
    • Increase aeration, as some medications can lower oxygen levels.

2. Isolation and Targeted Treatment

If you are concerned about treating the display tank, or if the infestation is severe:

  • Quarantine Tank (QT): Move the infected fish to a separate, bare-bottom quarantine or hospital tank.
    • Treat the fish with Praziquantel in the QT.
    • Freshwater Dips: A short freshwater dip (5 minutes maximum, with matching  and temperature) can cause the flatworms to drop off the fish immediately, providing quick, temporary relief. This should be followed by placement into a clean QT.
  • Fallow Period: Since the flatworms have a reproductive stage in the substrate of your main tank, the most certain way to eradicate them from your system is to leave the main tank fallow (fishless) for at least 6-8 weeks. Without a host, the worms will eventually die off.

3. Maintenance and Prevention

  • Deep Sand Bed: Because the flatworms encyst and reproduce in the substrate, you may want to thoroughly gravel vacuum your sand bed to remove as many cysts and organic debris (which the juvenile worms feed on) as possible.
  • Quarantine ALL New Fish: This is the single best preventative measure for all parasites. Quarantine all new fish in a separate tank for several weeks to observe and treat for any potential parasites (including both black and white ich) before introducing them to your main reef tank
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