Description
Fish Doxy (Fix Doxy) is doxycycline in 100 mg tablets from Aqua Soma Labs, for freshwater and saltwater aquarium fish. The label calls for one tablet per 15 gallons of water, dissolved and repeated every 24 hours for 5 to 10 days.
- Active ingredient
- Doxycycline
- Strength
- 100 mg per tablet
- Bottle sizes
- 30 or 60 tablets
- Label dose
- 1 tablet per 15 gal (57 L)
- Course
- Daily for 5 to 10 days
- Made by
- Aqua Soma Labs, USA
When aquarists choose fish doxycycline
Doxycycline belongs to the tetracycline family. It slows the growth of a wide range of bacteria, gram-positive and gram-negative alike, and it reaches tissue well. Keepers usually pick it when the problem looks deeper than ragged fins:
- Fin and tail rot that keeps coming back after a penicillin-type course
- Grey-white patches on the back or around the mouth (columnaris-type)
- Gills that look swollen or blotchy, with the fish gasping near the surface
- Red blotches under the skin and at the fin bases, often with clamped fins and lost appetite
Doxycycline won't help with ich, velvet, gill flukes, or fungus. White salt-like spots, flashing, and fluffy growths point to parasites or fungus, so treat those first. For hole-in-the-head and stringy white feces, look at Fish Zole instead.
Fish Doxy dosage by tank size
The Aqua Soma Labs label uses one 100 mg tablet for every 15 gallons. Some sellers quote one tablet per 10 gallons for other doxycycline brands, so follow the bottle you actually have.
| Tank | Liters | Tablets per day |
|---|---|---|
| 15 gal | 57 L | 1 |
| 30 gal | 114 L | 2 |
| 45 gal | 170 L | 3 |
| 60 gal | 227 L | 4 |
| 75 gal | 284 L | 5 |
| 90 gal | 341 L | 6 |
For a 10 gallon tank, crush one tablet into a measured cup of tank water and add two thirds of it. For 20 gallons, add one whole tablet plus a third of a second dissolved tablet. The label on your bottle takes priority over this chart.
Treating with Fish Doxy
- Set up a hospital tank. Use clean, dechlorinated water at the same temperature as the main tank, with a sponge filter and an air stone.
- Remove what absorbs it. Activated carbon, resins such as Purigen, and UV sterilizers all take doxycycline out of the water. Pull them for the whole course.
- Dissolve, then dose. Crush the tablets and stir them into a cup of tank water. Pour the mix across the surface so it spreads evenly.
- Change water, then re-dose daily. Replace part of the water each day before the next dose, and dose for the full volume again.
- Watch for 5 days. Appetite, breathing rate, and the edges of any patches tell you whether it's working. No change at all by day 5 means the cause probably isn't bacterial, or it needs a different antibacterial.
- Run the full course. Continue to day 10 if the fish are still recovering, then do a large water change and add fresh carbon to clear what's left.
30 or 60 tablets?
A 30-count bottle is exactly one 10-day course for a 45 gallon tank, or three courses in a 15 gallon hospital tank. Choose 60 tablets if your hospital tank is 60 gallons or larger, or if you want a second course on hand when a new group of fish arrives.
At list price a tablet costs $1.50 from the 30-count bottle and about $1.16 from the 60-count. A full 10-day course in a 30 gallon tank uses 20 tablets: about $30.00 from small bottles or $23.27 from the large one.
Fish Doxy next to other Fix antibacterials
| Product | Class | Coverage | Usually chosen for |
|---|---|---|---|
| Fish Doxy | Tetracycline (doxycycline) | Broad, gram-positive and gram-negative | Columnaris-type patches, gill problems, relapsing fin rot |
| Fish Mox | Penicillin type (amoxicillin) | Mainly gram-positive, some gram-negative | A first choice when the cause is unclear |
| Fish Zithro | Macrolide (azithromycin) | Broad, with a shorter course | Gill and fin infections over 5 to 7 days |
| Fish Flox | Fluoroquinolone (ciprofloxacin) | Mostly gram-negative | Fast, aggressive ulcers |
For the full lineup, see our guide to fish antibacterials.
Water and safety notes
- Doxycycline binds to calcium and magnesium, so very hard water can weaken it. Soft to moderately hard water gives the most consistent results.
- It breaks down faster under strong light. Keep the hospital tank lights low or off during treatment.
- A pale yellow tint in the water is normal and fades after the final water change.
- The biofilter can slow down. Test ammonia and nitrite daily and change water if either rises.
- Keep shrimp, snails, and corals out of the treatment tank.
- Store the bottle tightly closed, away from heat and light, and away from children and pets.
Fish Doxy questions
Is Fix Doxy the same thing as Fish Doxy?
Fish Doxy is the name most aquarists search for doxycycline tablets for fish. Fix Doxy is Aqua Soma Labs' doxycycline in the same 100 mg strength, so we list it under both names.
Is it one tablet per 10 or per 15 gallons?
For Fix Doxy the label says one 100 mg tablet per 15 gallons. Doses you read elsewhere may come from different products. Go by the bottle in your hand.
Are these tablets or capsules?
Tablets. Crush and dissolve them in tank water before adding them, so the dose spreads evenly instead of settling on the bottom.
Can I use Fish Doxy in a marine tank?
The label covers both freshwater and saltwater ornamental fish. In reef systems, treat the fish in a separate hospital tank so corals and invertebrates aren't exposed.
Where can I buy fish doxycycline?
Farm stores and big pet chains carry fish antibacterials on and off, and online listings often mix tablets, capsules, and different strengths. We stock Fix Doxy 100 mg in 30 and 60 count and ship from Missouri the same or next business day.
For aquarium and ornamental fish only.
