Seachem MetroPlex is a metronidazole-based aquarium treatment used primarily against the protozoan parasite Hexamita (Spironucleus) and the associated hole-in-the-head disease, as well as other internal protozoan and anaerobic bacterial infections. Metronidazole is a nitroimidazole that is highly effective against anaerobic organisms and certain protozoa, and it has long been a mainstay of treatment for discus, angelfish, cichlids, and marine tangs with hole-in-the-head disease. MetroPlex is sold as an over-the-counter aquarium product. Recent FDA enforcement actions have shifted metronidazole toward prescription status in some markets, so availability varies. For valuable fish, consult an aquatic veterinarian.

Overview

Metronidazole enters microorganisms and is reduced to reactive compounds that damage DNA, killing anaerobic bacteria and some protozoa. In aquarium fish it is most commonly used to treat Hexamita and Spironucleus — flagellated protozoans of the intestinal tract — and the related hole-in-the-head lesions that occur in discus, angelfish, oscars, and marine tangs. MetroPlex is formulated as a powder that can be dosed directly into aquarium water or bound to food using Seachem Focus for targeted delivery to the gut.

Uses

  • Hexamita and Spironucleus infections
  • Hole-in-the-head disease (lateral line erosion) in cichlids and tangs
  • Internal protozoan parasites
  • Adjunctive treatment for ich
  • Some anaerobic bacterial infections
  • Cryptocaryon when combined with other treatments

Dosage

  • Water dosing: One level measure (included scoop) per 10 gallons every 2 days, for a minimum of 2 weeks
  • Food dosing (preferred for internal infection): Mix 1 measure MetroPlex with 1 measure Seachem Focus per tablespoon of food. Feed for 5 to 10 days.
  • Hole-in-the-head treatment: Combined water and food dosing is often most effective
  • Perform a 25% water change before redosing

Always follow the product label for exact dosing.

Safety Considerations

  • Invertebrate-safe — MetroPlex is generally considered safe for shrimp, snails, and corals at label dose
  • Reef-safe — Can be used in reef tanks for treating marine fish with internal parasites
  • Scaleless fish — Generally well-tolerated in scaleless species at standard dose
  • Biological filter — Metronidazole can affect anaerobic denitrifying bacteria. Monitor ammonia and nitrite during extended treatment.
  • Light sensitivity — Metronidazole degrades in light. Dim tank lighting during treatment.
  • User safety — Metronidazole should not be handled by pregnant individuals. Wear gloves during dosing.
  • Regulatory status — FDA has moved metronidazole toward prescription status. Availability as an OTC aquarium product may change.

Water Chemistry Effects

  • Minimal effect on pH, hardness, or salinity
  • Degrades in light — treatment may be less effective in brightly lit tanks
  • Activated carbon removes metronidazole. Remove carbon during treatment.
  • Does not significantly affect nitrifying filter bacteria at label dose

How to Use

  1. Test water parameters and perform a water change if needed
  2. Remove activated carbon from the filter
  3. Dim the aquarium lights
  4. For water dosing: dissolve one scoop in a small amount of tank water and distribute around the tank
  5. For food dosing: mix MetroPlex with Seachem Focus and bind to the fish’s regular food
  6. Continue treatment for at least 2 weeks
  7. After treatment, perform a 25% water change and return activated carbon

FAQs

Is MetroPlex reef-safe? Yes, it is considered safe for reef tanks and invertebrates at label dose.

Can I mix it with Kanaplex? Yes, MetroPlex and Kanaplex are often combined with Seachem Focus and fed together for hole-in-the-head disease.

Will it affect my biological filter? At label dose the impact on nitrifying bacteria is minimal. Monitor parameters during extended treatment.

Is it effective for ich? Metronidazole has some activity against ich but is not as effective as dedicated ich treatments. It is sometimes added to a treatment plan for ich in cichlids.

Do I need a prescription? In the United States, FDA enforcement has limited OTC availability of fish metronidazole. Check current availability in your market.