Description
Fish Zithro (Fix Zithro) is fish azithromycin in 250 mg tablets from Aqua Soma Labs, for freshwater and saltwater aquarium fish. The label dose is one tablet per 10 gallons, dissolved and added every 24 hours for 5 to 7 days, a shorter course than most of the Fix line.
- Active ingredient
- Azithromycin
- Strength
- 250 mg per tablet
- Bottle sizes
- 12 or 30 tablets
- Label dose
- 1 tablet per 10 gal (38 L)
- Course
- Once a day for 5 to 7 days
- Made by
- Aqua Soma Labs, USA
What azithromycin does for aquarium fish
Azithromycin is a macrolide. It stops bacteria from building proteins, reaches gram-positive and a good share of gram-negative species, and stays in fish tissue longer than most antibacterials. That's why its course is shorter. Aquarists choose Fish Zithro for:
- Fin and tail rot that has moved past the fin edges toward the body
- Columnaris-type infections: pale saddle-shaped patches, frayed mouths, or cottony lips
- Gill infections with pale or mottled gills, fast breathing, and fish hanging near the surface
- Cases where a 10-day course is hard to manage and a 5 to 7 day course is easier on the fish
Azithromycin isn't for parasites or fungus. Rule out ich, velvet, and gill flukes first, since gill flukes look a lot like a bacterial gill problem.
Fish Zithro dosage by tank size
| Tank | Liters | Tablets per day | Tablets for a 7-day course |
|---|---|---|---|
| 10 gal | 38 L | 1 | 7 |
| 20 gal | 76 L | 2 | 14 |
| 30 gal | 114 L | 3 | 21 |
| 40 gal | 151 L | 4 | 28 |
| 55 gal | 208 L | 5½ | 39 |
Some sellers quote a 5 to 6 day course. The Aqua Soma Labs label says 5 to 7 days, and the label on your bottle is what to follow. In tanks smaller than 10 gallons, break a tablet, or dissolve a whole one in measured water and pour in your share.
A Fish Zithro course, day by day
- Setup. Hospital tank, sponge filter, extra aeration. Carbon, resins, and UV out.
- Day 1. Crush and dissolve the tablets in a cup of tank water and add them evenly. Expect the fish to stay quiet.
- Days 2 and 3. Change part of the water before each dose. Breathing should start to slow and patches should stop spreading.
- Days 4 and 5. Most fish start eating again. If nothing has changed by day 5, stop and reconsider the cause.
- Days 6 and 7. Finish the course while the fish improve. Don't stop at day 5 just because the patches have faded.
- After day 7. Make a large water change and add fresh carbon. Feed lightly for a few days while the filter recovers.
12 or 30 tablets?
The 12-count bottle is one 6-day course in a 20 gallon tank, or a full week in a 10 gallon one. The 30-count bottle covers a 7-day course in a 40 gallon tank, and it's the cheaper choice per tablet. At list price a 7-day course in 20 gallons costs about $35.40 from the 30-count bottle.
Fish Zithro compared
| Product | Class | Course | Typical use |
|---|---|---|---|
| Fish Zithro | Macrolide (azithromycin) | 5 to 7 days | Columnaris-type and gill infections |
| Fish Doxy | Tetracycline (doxycycline) | 5 to 10 days | Similar cases in soft water; relapsing fin rot |
| Fish Mox | Penicillin type | 5 to 10 days | Broad first choice for fin rot and sores |
| Fish Flox | Fluoroquinolone | 5 to 10 days | Aggressive gram-negative ulcers |
See the full Fix line on our fish antibacterials page.
Safety notes
- Azithromycin can disrupt nitrifying bacteria. Test ammonia and nitrite every day.
- Keep invertebrates and corals out of the hospital tank.
- Columnaris spreads faster in warm water. Avoid raising the temperature during treatment.
- Don't combine Fish Zithro with other treatments without an aquatic veterinarian's advice.
- Store sealed and dry, away from children and pets. Don't flush unused tablets.
Fish Zithro questions
Fish Zithro or Fix Zithro?
Fish Zithro is the name aquarists search for azithromycin tablets for fish. Fix Zithro is the Aqua Soma Labs 250 mg tablet, so this page uses both names.
Why is the course shorter than other fish antibacterials?
Azithromycin stays in tissue for a long time, so the label course is 5 to 7 days instead of up to 10.
Can I use it in saltwater?
Yes. The label covers freshwater and saltwater ornamental fish. Treat reef fish away from corals and invertebrates.
How do I know it's working?
By day 3 the patches should stop growing and breathing should slow. By day 5 most fish eat again. No change at all by day 5 means stop and rethink.
Where can I buy fish azithromycin?
Stock in local stores is unpredictable. We carry Fix Zithro 250 mg in 12 and 30 count and ship from Missouri the same or next business day.
For aquarium and ornamental fish only.
