KanaPlex vs MetroPlex: Which Seachem Fish Medication Do You Need?

April 10, 2026

If you've ever stood in front of a sick fish wondering which Seachem medication to grab, you're not alone. KanaPlex and MetroPlex are two of the most popular and effective treatments in the freshwater aquarium hobby — but they treat completely different problems, and using the wrong one can waste time while your fish gets sicker.

This complete guide from Tropical Treasures Wyo in Cheyenne, Wyoming breaks down exactly what each medication does, when to use each one, how to dose them safely, and when (and how) you can use them together. By the end you'll know exactly which medication your fish needs.

💊 Quick Answer

KanaPlex is a broad-spectrum antibiotic. Use it for bacterial infections — fin rot, columnaris, popeye, dropsy, septicemia, and gram-negative infections.
MetroPlex is an antiprotozoal/antiparasitic. Use it for internal parasites and protozoans — Hexamita, Hole-in-the-Head disease, internal flagellates, and Spironucleus.
They treat different organisms, so they can be safely combined when your fish has both bacterial and parasitic symptoms.

💊 What Is KanaPlex?

Seachem KanaPlex is a kanamycin-based broad-spectrum antibiotic that targets both gram-positive and gram-negative bacteria. It's one of the most reliable treatments in the hobby for serious bacterial infections that don't respond to milder treatments.

What KanaPlex Treats

Fin and tail rot — frayed, discolored, or receding fins
Columnaris — white/gray patches, "saddleback" lesions, mouth fungus appearance
Popeye — bulging eyes from bacterial infection
Dropsy — pinecone-like raised scales (early stage)
Bacterial septicemia — red streaks in fins or body
Open ulcers and sores
Some external bacterial infections on the skin and gills

How KanaPlex Works

Kanamycin is absorbed through both the gills and the gut, which makes KanaPlex effective whether dosed in tank water or fed to the fish via a medicated food (more on that below with Seachem Focus). It's potent and works best in a hospital tank to avoid harming biological filtration in your main display.

💊 What Is MetroPlex?

Seachem MetroPlex contains metronidazole, a powerful treatment for protozoan parasites and certain anaerobic bacteria. It's especially effective against the internal parasites that cause "Hole-in-the-Head" disease in cichlids and chronic wasting in many freshwater species.

What MetroPlex Treats

Hexamita — the protozoan behind Hole-in-the-Head disease
Spironucleus — internal flagellate parasites
Internal protozoan infections causing white stringy feces
Chronic wasting with normal eating (a classic internal parasite sign)
Some anaerobic bacterial infections
Marine and freshwater Ich in some protocols (as a supporting treatment, not first-line)

How MetroPlex Works

Metronidazole works inside the fish's digestive system, which is exactly where most of these protozoan parasites live. That's why combining it with Seachem Focus as a medicated food is the gold standard for internal infections — it delivers the active ingredient directly where the parasites are.

⚖️ Side-by-Side Comparison

Feature	KanaPlex	MetroPlex
Active ingredient	Kanamycin sulfate (antibiotic)	Metronidazole (antiprotozoal)
Treats	Bacterial infections	Protozoan parasites & anaerobes
Common uses	Fin rot, columnaris, popeye, dropsy	Hexamita, HITH, internal flagellates
Route	Water + feed	Water + feed (best as feed)
Dose (tank)	1 measure per 5 gallons every 48 hrs	1 measure per 10 gallons every 24 hrs
Treatment length	Up to 3 doses (6 days)	Up to 3 doses (3 days) or as feed for 5–7 days
Reef-safe	No (biological filter risk)	Generally yes at low doses
Hospital tank?	Strongly recommended	Recommended

🧠 When to Use KanaPlex

Reach for KanaPlex when symptoms point to a bacterial problem:

Fins are frayed, ragged, or shrinking despite clean water
White or gray patches on the body or mouth (columnaris)
One or both eyes are visibly bulging
Red streaks running through fins or along the body
Open sores, ulcers, or "wounds" that won't heal
Early-stage dropsy (raised scales but fish still eating)

For external skin and fin infections, dose KanaPlex directly in the hospital tank water. For systemic infections, combine it with Seachem Focus and feed as a medicated food (see below).

🧠 When to Use MetroPlex

Reach for MetroPlex when symptoms point to a parasitic or protozoan problem:

Pitting or holes on the head and along the lateral line (especially in cichlids, oscars, discus, angelfish)
White, stringy, or mucus-filled feces
Chronic weight loss despite normal eating
Loss of appetite combined with hiding and color loss
Flashing or rubbing in the absence of obvious external parasites
"Wasting" or hollow-belly appearance

MetroPlex is most effective fed in food via Focus — internal parasites live in the gut, and metronidazole needs to reach them there.

💡 Can You Use KanaPlex and MetroPlex Together?

Yes — and it's actually a recommended combination for serious or unclear cases. Because the two medications target completely different organisms (bacteria vs. protozoans), they don't compete or interfere with each other. The classic Seachem treatment trio combines:

KanaPlex (antibiotic)
MetroPlex (antiprotozoal)
Focus (medication binder for food)

The Seachem Medicated Food Recipe

Take a small amount of fish food (enough for one feeding).
Mix in 1 measure (¼ tsp) of Focus + 1 measure of KanaPlex and/or MetroPlex.
Add a few drops of tank water to bind the medication to the food.
Feed within 5 minutes — repeat 1–2 times daily for 5–7 days.

This medicated food approach delivers far higher concentrations of medication directly to the infected fish than dosing the tank water alone, and it's much gentler on biological filtration. For fish that are still eating, it's almost always more effective than tank dosing.

🏥 Hospital Tank Protocol

Whenever possible, treat sick fish in a separate hospital tank rather than your main display:

Use a 10–20 gallon tank with a heater, sponge filter, and minimal decor.
Match temperature and pH to the main tank.
Add Seachem Prime for any fresh water added.
Skip substrate and live plants — both can absorb medications and reduce effectiveness.
Do a 25–50% water change between doses to keep parameters clean.

This protects your main tank's biological filter from antibiotics, prevents stressing healthy tankmates, and keeps medication concentrations accurate. New to cycling a quarantine tank? See our nitrogen cycle guide for setup.

⚠️ Common Mistakes to Avoid

Treating in the main tank without removing carbon — activated carbon strips medication out of the water within hours.
Stopping treatment early — when symptoms improve, finish the full course or the infection comes back stronger.
Combining with copper-based meds — never use these treatments alongside copper if you have invertebrates or scaleless fish.
Using KanaPlex on a healthy biological filter — it can crash your nitrogen cycle. Always treat in a hospital tank.
Guessing at the diagnosis — review symptoms carefully in our common fish diseases & treatment guide before reaching for any medication.
Skipping water quality testing — many "diseases" are actually ammonia or nitrite poisoning. Always test water first.

🔍 Frequently Asked Questions

Are KanaPlex and MetroPlex safe for shrimp and snails?

MetroPlex is generally considered invertebrate-safe at standard doses. KanaPlex can be harmful to invertebrates and shouldn't be used in tanks with shrimp or sensitive snails — always treat in a separate hospital tank.

Will these medications harm my biological filter?

KanaPlex can significantly impact nitrifying bacteria, which is why a hospital tank is strongly recommended. MetroPlex is much gentler on biological filtration but can still cause minor disruption.

How long should I treat my fish?

The standard course is 3 doses over 6 days for KanaPlex and 3 doses over 3 days for MetroPlex (or 5–7 days as medicated food). Always finish the full course even if symptoms improve.

Can I dose KanaPlex and MetroPlex in the same tank at the same time?

Yes. They have different mechanisms and don't interact. Many breeders use both simultaneously when fish show signs of both bacterial and parasitic infection.

What's the difference between MetroPlex and Fritz MetroCleanse?

Both contain metronidazole and treat the same conditions. Fritz MetroCleanse comes in pre-measured packets, which makes dosing easier. MetroPlex offers more flexibility for medicated food preparation.

What about ParaGuard or NeoPlex — when do I use those instead?

Seachem ParaGuard is a milder, broad-spectrum treatment for external parasites (like Ich and flukes) and external bacterial/fungal infections. Seachem NeoPlex uses neomycin and is another antibiotic option, often used for stubborn gram-negative infections.

Can I use these in my reef tank?

MetroPlex is generally reef-safe at standard doses; KanaPlex is not. Always treat reef fish in a separate hospital tank.

My fish stopped eating. Will medicated food still work?

If your fish won't eat, switch to in-tank water dosing. Once appetite returns, transition to medicated food for stronger results.

🛒 Treat Your Fish the Right Way

Browse our complete medication selection at Tropical Treasures Wyo:

Not sure what's wrong with your fish? Contact Tropical Treasures Wyo at 307-369-1118 or visit our shop at 190 S College Drive, Suite D, Cheyenne, WY 82007. We help diagnose and recommend the right treatment, and we ship fish medications nationwide.

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