Your angelfish is hanging near the surface, turning dark, and leaving pale, stringy waste behind. Before you reach for a bottle, it helps to know what you may be dealing with. This guide covers hexamita (more precisely, Spironucleus), how to spot it, and where Fish Zole, a metronidazole tablet made for aquarium fish, fits into the plan.
General information, not veterinary advice. Medications on this page are for animal use only; follow the product label for dosing and ask an aquatic veterinarian if you're unsure.
Key Takeaways
On this page
New to fish medications overall? Our guide to which aquarium medication fits which problem sets the big picture first.
In 2015, University of Florida IFAS Extension authors Ruth Francis-Floyd and Roy Yanong noted that Spironucleus is the main pathogenic group in fish, while true Hexamita species are mostly free-living and non-parasitic. So "hexamita" is hobby shorthand for an intestinal flagellate that can hurt your fish.
According to the 2015 UF/IFAS Extension guide VM 67, Spironucleus is the pathogenic group in freshwater aquarium fish, and it usually spreads when fish eat parasites shed in contaminated feces. The guide also says fish often carry it in small numbers without getting sick.
That last point is the one most fish keepers miss. The parasite can sit quietly in a healthy fish. It's the stress that tips the balance.
Researchers studying Spironucleus vortens worked with a strain isolated from an angelfish and describe the species as causing major problems in ornamental fish aquaculture (Ode Millet, Cable and Lloyd, 2010). Plenty of home aquariums run into the same thing, usually without a name for it.
The UF/IFAS guide lists weight loss, darkened skin, reduced activity and refusal of food as the classic signs. In badly affected angelfish, it describes fish floating horizontally at the surface with swollen bellies, plus white or pale, stringy feces.
The 2015 UF/IFAS guide VM 67 lists weight loss, dark skin, lethargy, food refusal and pale, stringy feces as signs of Spironucleus infection. It also warns that other diseases can look the same, so a lab or microscope check is the only way to be sure what you're treating.
Hole-in-the-head signs deserve a note. One published oscar case report describes lost appetite, bulging eyes and darkened skin before holes appeared on the head, with heavy flagellate infection in lesion smears (case report on hole in the head in an oscar). The same fish also grew large numbers of Aeromonas hydrophila bacteria, so mixed infections happen.
Start your investigation with the water. Test strips and a reliable thermometer take the guesswork out of the first check, and the water testing and common diseases primer walks through what the numbers mean.
7 Way Aquarium Test Strips, 125 Count
Tests chlorine, hardness, carbonate, alkalinity, nitrate, nitrite and pH.
$22.04 Shop Now →
9 in 1 Aquarium Test Strips, 100 Count
Nine-parameter strips for salt and freshwater tanks, 100 count.
$65.91 Shop Now →
The UF/IFAS guide names cichlids such as angelfish, oscars and discus as especially susceptible. It points to stress from poor nutrition, shipping, overcrowding, poor water quality or other infections as the trigger that lets the parasite multiply fast.
In its 2015 guide, UF/IFAS Extension says angelfish, oscars and discus are especially susceptible to Spironucleus. Stress from shipping, crowding, poor feeding or poor water quality lets low-level parasites multiply quickly, which is why husbandry matters as much as medication in any recovery plan.
Here's the practical read. A freshly shipped angelfish in a crowded, unfiltered tank is the textbook setup. A calm, planted tank with steady temperature and good filtration takes much of that pressure off.
Plants give shy cichlids cover, and a quiet, well-filtered tank keeps water parameters steady.
XXL Aquarium Plants, Assorted Colors
Assorted-color XXL plants to dress up an aquarium.
$17.79 Shop Now →
LED Planted Tank Light, 4 Channels
Four-channel LED light with remote control for planted tanks.
$64.63 Shop Now →
UF/IFAS says diagnosis relies on a fresh intestinal tissue squash examined under a light microscope at 200x and 400x. When the fish can't be sacrificed, a wet mount of fresh feces gives a fairly good estimate of parasite load.
The UF/IFAS Extension guide VM 67 (2015) says Spironucleus is diagnosed with a fresh intestinal squash viewed at 200x and 400x. If the fish must be kept alive, a wet mount of fresh feces is a fairly good way to estimate how heavy the infection is.
Is a microscope realistic at home? Not for most of us. Your local aquatic veterinarian, fish store with a lab partner, or a university extension service can often run a fecal wet mount. It costs far less than guessing and treating the wrong problem.
Do you suspect bacteria rather than parasites? The Fish Dox guide covers where that antibiotic fits, and the medication comparison article above helps you sort the options.
UF/IFAS identifies metronidazole as the treatment of choice for Spironucleus in freshwater aquarium fish. Fish Zole is a 250mg metronidazole tablet made for aquarium fish, and it's the item to look at when a confirmed intestinal flagellate is the problem.
The 2015 UF/IFAS guide calls metronidazole the treatment of choice for Spironucleus. A separate in vitro study of Spironucleus vortens from angelfish found metronidazole among the agents that inhibited parasite growth, with markedly fewer parasites at concentrations of 1 microgram per milliliter or more (Diseases of Aquatic Organisms, vol. 38).
A few tips for using it well:
Views differ on bath versus medicated feed: the UF/IFAS guide notes a bath may clear the parasite less effectively than medicated feed, which is one more reason to match the method to how well your fish is eating.
Fish Zole 250mg Tablet, 30 Count
Metronidazole aquarium fish tablets, 30 count. For animal use only.
$33.99 Shop Now →
Fish Mox 250mg Capsules
Aquarium fish capsules for the fish-keeper medicine shelf. Animal use only.
$20.99 Shop Now →
Fish Dox 100mg Capsules, 30 Count
Aquarium fish capsules, 30 count. Animal use only; follow the label.
$43.99 Shop Now →
UF/IFAS recommends quarantining, diagnosing and treating new fish before they join existing stock. A hospital tank also keeps medicated water away from healthy fish and live plants, and it makes the big water changes that bath treatments call for much easier.
UF/IFAS Extension advises quarantining, diagnosing and treating new fish before adding them to existing stock (2015). A bare hospital tank with a heater, gentle filtration and easy water changes keeps medicated water contained and lets you watch appetite and feces closely.
Our step-by-step quarantine hospital tank guide covers the gear list. A heater, a thermometer and a gentle filter are the core.
Mini Flat Heater for Small Aquariums
Compact flat heater sized for small aquariums.
$13.18 Shop Now →
Submersible Aquarium Heater, to 55 Gallon
Submersible heater for tanks up to 55 gallons.
$39.87 Shop Now →
Aquarium Power Filter, 3 Cartridges
Power filter that comes with three cartridges.
$17.52 Shop Now →
Prevention follows the UF/IFAS advice. Quarantine new fish, clean gravel and filter media thoroughly, assume the parasite lives wherever organic material builds up, and improve water quality, nutrition and husbandry to reduce stress.
UF/IFAS Extension says to assume Spironucleus is present wherever organic material has built up, and to clean gravel and filter media thoroughly. Quarantining new fish and improving water quality, nutrition and husbandry reduce the stress that lets the parasite multiply (2015).
Here's a simple routine. Condition new tap water, test weekly, vacuum debris before it piles up, and feed measured portions. If a tank has a rough week, ammonia control and stress support help buy time.
Stress Zyme, Freshwater and Saltwater
Aquarium treatment for freshwater and saltwater tanks.
$16.68 Shop Now →
Ammo Lock Ammonia Detoxifier, 1 Gallon
Ammonia detoxifier for fresh and saltwater aquariums, 1 gallon bottle.
$60.43 Shop Now →
Entry picks
Fin Pen 250mg Tablet, 30 Count
Aquarium fish tablets, 30 count. Animal use only; follow the label.
$23.99 Shop Now →
Fish Sulfa 960mg Tablets, 30 Count
Aquarium fish tablets, 30 count. Animal use only; follow the label.
$32.95 Shop Now →
Fish Ceph 500mg Capsules, 60 Count
Aquarium fish capsules, 60 count. Animal use only; follow the label.
$38.49 Shop Now →
Fish Cipro 500mg Tablets, 30 Count
Aquarium fish tablets, 30 count. Limited stock. Animal use only.
$46.99 Shop Now →
Mid-range picks
Power Filter, 3-Step, 30-60 Gallons
Three-step filtration for 30 to 60 gallon aquariums.
$78.99 Shop Now →
Glass 29 Gallon Starter Kit, Black
Glass 29 gallon aquarium starter kit in black.
$151.24 Shop Now →
20 Gallon Aquarium Starter Kit, LED
Glass 20 gallon starter kit with LED lighting.
$196.29 Shop Now →
Premium picks
90g Corner Reef-Ready Aquarium Set, White
Glass 90 gallon corner 1/4 cylinder aquarium set. One in stock.
$4,789.99 Shop Now →
Not exactly. Hobbyists say hexamita for the intestinal flagellate behind hole-in-the-head and wasting signs, but the UF/IFAS extension guide says true Hexamita species are mostly free-living and non-parasitic, and that Spironucleus is the main pathogenic group in fish. The treatment conversation is the same either way.
Fish Zole is a 250mg metronidazole tablet made for aquarium fish. Metronidazole is the treatment of choice named in the UF/IFAS guide for Spironucleus infections. It's for animal use only, so follow the product label for the exact amount, schedule and water changes.
You can, but a separate quarantine tank is usually easier to manage. A bare hospital tank keeps medicated water or food contained, makes water changes simple, and protects healthy tankmates and live plants. Our quarantine tank guide covers the gear.
The UF/IFAS guide lists angelfish, oscars and discus among the susceptible cichlids. The parasite often lives in fish in small numbers and only blooms when stress hits, such as shipping, overcrowding, poor nutrition or poor water quality. Steady water and good feeding are part of the answer.
Call an aquatic or exotics veterinarian if signs persist after treatment, if several fish are failing at once, or if you're unsure what you're seeing. The UF/IFAS guide notes that similar signs can come from other diseases, and a fresh fecal wet mount under a microscope is the clearest way to confirm.
Sources: (1) University of Florida IFAS Extension, Spironucleus Infestations (Spironucleosis) in Freshwater Aquarium Fish (VM 67, Francis-Floyd and Yanong, rev. 2015), journals.flvc.org, retrieved 2026-10-08. (2) Journal of Eukaryotic Microbiology (Ode Millet, Cable and Lloyd, 2010), The diplomonad fish parasite Spironucleus vortens produces hydrogen, orca.cardiff.ac.uk, retrieved 2026-10-08. (3) Inter-Research, Diseases of Aquatic Organisms vol. 38(1) pp. 47-52, abstract page on in vitro agents against Spironucleus vortens, int-res.com, retrieved 2026-10-08. (4) Iranian veterinary journal site (ijvst.um.ac.ir), Case Report and Treatment of Hole in the Head in Oscar, Astronotus ocellatus, ijvst.um.ac.ir, retrieved 2026-10-08.
$220.53
$47.32
$52.99
$23.45
$129.45
$220.53
We read four airline pet pages to see how big a wheeled carrier can be, whether wheels matter, and how to measure before you buy.
A practical guide to bark collars for dogs: how vibration, spray, static, and ultrasonic types work, how to fit one, and how to use it kindly after finding the cause of the barking.
