Fish Flex (Fix Flex) Cephalexin 250mg & 500mg Capsules

Fish Flex (Fix Flex) is fish cephalexin in 250 mg and 500 mg capsules from Aqua Soma Labs, for freshwater and saltwater aquarium fish. Per the label, dissolve 250 mg for every 10 gallons or 500 mg for every 20 gallons, once a day for 5 to 10 days.

Active ingredient

Cephalexin

Strengths

250 mg or 500 mg per capsule

Bottle sizes

30 or 100 capsules

Label dose

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

Course

Daily for 5 to 10 days; stop if no improvement by day 5

Made by

Aqua Soma Labs, USA

Why keepers reach for fish cephalexin

Cephalexin is a first-generation cephalosporin. Like amoxicillin it attacks the bacterial cell wall, but it holds up against some bacteria that break down penicillin-type antibacterials. That makes Fish Flex a common second step when amoxicillin didn't clear a problem, and a first pick for skin damage:

• Open sores and red ulcers on the flanks, often after a fight or a scrape on decor
• Wounds that turn white or fuzzy-edged instead of closing
• Fin rot with red, inflamed fin bases
• One cloudy, swollen eye after an injury

Cephalexin won't touch parasites or fungus. Rule out ich, velvet, and flukes before you start, and use Fish Zole for internal flagellates.

Fish Flex dosage by tank size

Fractions come from splitting the powder of an opened capsule, so treat them as close estimates. The 250 mg capsule gives you finer control in small tanks. If your bottle's label differs from this chart, follow the label.

Using Fish Flex

1. Separate the sick fish. A hospital tank with a sponge filter and gentle aeration keeps the dose accurate and protects your display tank.
2. Pull the carbon. Remove activated carbon and resins and turn off UV for the whole course.
3. Pre-mix every dose. Empty the capsules into a cup of tank water, stir until no powder is floating, and add it across the tank.
4. Once a day, change and re-dose. Swap part of the water, then add a full dose for the tank's volume.
5. Judge it on day 5. Wounds should look cleaner and less red. If there's no improvement at all, the label says to stop.
6. Close out. Finish the course while the fish improve, then do a big water change and put carbon back in to pull out leftover cephalexin.

Which bottle to buy

The 500 mg capsule is what many sellers call Fish Flex Forte. It's the better value once your hospital tank holds 20 gallons or more. The 250 mg capsule is easier to dose in small tanks and bowls.

From the 100-count bottles, a 10-day course in a 20 gallon tank costs about $7.00 with 250 mg capsules or $5.00 with 500 mg. In a 40 gallon tank it's about $14.00 or $10.00. A 100-count bottle of either strength covers several courses, which is handy if you quarantine every new fish.

Cephalexin or amoxicillin?

For deep wounds and mouth rot, compare Fish Cin. For gram-negative ulcers that spread fast, look at Fish Sulfa Forte. Every option is on our fish antibacterials page.

Before you dose

• Cephalexin can slow the nitrifying bacteria in your filter. Test ammonia and nitrite each day.
• Keep shrimp, snails, and corals out of the treated water.
• Clean, stable water matters as much as the antibacterial for wounds to close. Keep temperature steady and nitrate low.
• Don't combine Fish Flex with other treatments without advice from an aquatic veterinarian.
• Store capsules sealed in a cool, dry spot away from children and pets, and don't flush leftovers.

Fish Flex questions

Related guide: Ulcers and red sores on fish

For aquarium and ornamental fish only.