Fish Zole (Fix Zole) Metronidazole 250mg & 500mg Tablets

Fish Zole (Fix Zole) is metronidazole in 250 mg and 500 mg tablets from Aqua Soma Labs, for freshwater and saltwater aquarium fish. Following the label, you add 250 mg per 10 gallons or 500 mg per 20 gallons each day for 5 to 10 days, and no longer than 10.

Active ingredient

Metronidazole

Strengths

250 mg or 500 mg per tablet

Bottle size

60 tablets

Label dose

250 mg per 10 gal (38 L), or 500 mg per 20 gal (76 L)

Course

Once a day for 5 to 10 days, never past day 10

Made by

Aqua Soma Labs, USA

What fish metronidazole is for

Metronidazole is different from the rest of the Fix line. It works on protozoan flagellates, the single-celled parasites that live in a fish's gut, and on anaerobic bacteria. Most other fish antibacterials do nothing against flagellates. Keepers use Fish Zole when they see:

• Small pits or eroded spots on the head and along the lateral line (hole-in-the-head)
• White, stringy, or clear feces with a fish that stops eating
• Weight loss or a pinched belly even though the fish still tries to feed
• Bloat in cichlids when internal flagellates are the suspected cause
• Discus or angelfish that darken, hide, and refuse food, a common picture with Hexamita and Spironucleus

It's not the bottle to reach for with ich, velvet, gill flukes, or Costia, and it isn't a first choice for typical fin rot or skin ulcers. Those bacterial cases respond better to Fish Mox or Fish Flox.

Fish Zole dosage by tank size

Tablets can be broken in half; for smaller fractions, dissolve a whole tablet in a measured amount of water and pour in the share you need. Base the dose on the water in the tank, not on how many fish are sick. Your bottle's label overrides this chart.

A Fish Zole treatment, day by day

1. Before day 1. Move affected fish to a hospital tank if you can. Remove activated carbon and chemical media, keep the sponge filter running, and add extra aeration.
2. Day 1. Crush the tablets, dissolve them in a cup of tank water, and add the mix evenly across the surface.
3. Days 2 to 5. Change part of the water each day, then dose again for the full volume. Look for the fish to start picking at food and for feces to turn dark and solid again.
4. Day 5 check. If nothing has improved, stop. Another cause, or a fish too weak to recover, is more likely than an underdose.
5. Days 6 to 10. Keep dosing daily while the fish are improving. Don't go beyond day 10.
6. After the course. Make a large water change, return the carbon, and feed small amounts of high-quality food while the fish regain weight.

250 mg or 500 mg?

Both bottles hold 60 tablets of the same metronidazole. The 250 mg tablet suits 10 to 30 gallon hospital tanks, where half tablets of the 500 mg would be fiddly. Above 40 gallons the 500 mg tablet means half as many to crush and costs less per dose.

In a 20 gallon tank a 10-day course uses 20 tablets of 250 mg, about $14.00 at list price, or 10 tablets of 500 mg, about $10.00. In a 40 gallon tank the same course is about $28.00 with 250 mg tablets or $20.00 with 500 mg. If you've used metronidazole powders before, the active ingredient is the same; tablets just come pre-measured.

Fish Zole and the bacterial Fix products

Not sure which one fits? Start with our overview of fish antibacterials.

Tank and safety notes

• Metronidazole tends to be easier on the biofilter than broad antibacterials, but test ammonia and nitrite during the course anyway.
• Hole-in-the-head is often tied to water quality. Fix nitrate levels and filtration while you treat, or the pits come back.
• Keep invertebrates and corals out of the treatment water.
• Don't mix Fish Zole with other treatments unless an aquatic veterinarian tells you to.
• Store the bottle sealed, dry, and away from children and pets. Don't flush unused tablets or treated water.

Fish Zole questions

Related guide: How to set up a hospital tank for fish antibacterials

For aquarium and ornamental fish only.