Sick Longfin Danio

Request Help: Is this condition treatable, or is the fish on a slow terminal decline? Is it a danger to the rest of the tank?

Tank size: 75 Gal with Fluval FX4 filter (250 Gal capacity)
pH: 7.7-7.8
ammonia: 0.0
nitrite: 0.0
nitrate: ~ 20 ppm (15 Gal w/c per week)
kH/gH: unknown
tank temp: 76 deg F

Fish Symptoms: Fish is listless hanging near the surface. Seems to struggle swimming (jerky movement, not sinuous). Appears emaciated and bent; fins are ragged (pectoral, pelvic, caudal). Gills appear red in photo, harder to notice visually. Dorsal & Pectoral fins may be clamped. I've tried 1-hr dips each of the past two weekends in Seachem Paraguard, which didn't make any obvious impact positive or negative.
Volume and Frequency of water changes: 15 Gal w/c per week
Chemical Additives or Media in your tank: 3 ml API Algaefix used per week with w/c to deal with green algae
Tank inhabitants: 1- Longfin Zebra Danio, 1-Black Neon Tetra, 3-bandit cory, 2-albino cory, 5-harlequin raspbora, 1-bristlenose pleco, 4-dwarf gourami, 6-black skirt tetra, 6 glowlight tetra, 6 bloodfin tetra. Live plants. Assassin snails
Recent additions to your tank (living or decoration): none
Exposure to chemicals: none

I'm no expert. But he looks like he has a broken spine, or he is dying of old age??

How old is he?

How long have you had the fish for?
How long has it been like that for?
Can you post a short video of the fish swimming?

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Adding algicide every water change can be a contributing factor to fish health issues. Algicides are poisonous and affect fish as well as plants. If you have algae problems, either increase water changes or decrease light.

Stop using the algae killer.

Giving fish dips/ baths is not a good way to treat them. Every time you catch the fish and lift it out of the water, you can damage it. If you need to treat a fish, either treat the main tank it is in, or move the fish into a quarantine tank and treat it there.

The fish looks like it has gill flukes, which would cause the flared gills, emaciated body and anemia. Algicide poisoning could also be a contributing factor to the flared gills.

The fish could have intestinal worms, which would cause the skinny body and anemia.

The bent body is a concern and could be from old age, an internal growth or something else.


You can use Praziquantel to treat tapeworm and gill flukes. And Levamisole to treat thread/ round worms. If you can't find these medications, look for Flubendazole, which treats both lots of worms. I would try praziquantel first.

Remove carbon from filters before treatment and increase aeration/ surface turbulence to maximise oxygen levels in the water.

You treat the fish once a week for 4 weeks. The first treatment will kill any worms in the fish. The second, third and forth treatments kill any baby worms that hatch from eggs inside the fish's digestive tract.

Treat every fish tank in the house at the same time to prevent cross contamination.
You do a 75% water change and complete gravel clean 24-48 hours after treatment. Clean the filter 24 hours after treatment too.

Do not use the 2 medications together. If you want to treat both medications in a short space of time, use Praziquantel on day one. Do a 75% water change and gravel clean the substrate on day 2 & 3. Treat the tank with Levamisole on day 4 and do a 75% water change and gravel clean on day 5, 6 & 7 and then start with Praziquantel again on day 8.

The water changes will remove most of the medication so you don't overdose the fish the next time you treat them. The gravel cleaning will suck out any worms and eggs that have been expelled by the fish. Repeating the treatment for 3-4 doses at weekly intervals will kill any worms that hatch from eggs. At the end of the treatment you will have healthier fish.

Colin_T, thank you for the advice.
To answer your questions, I've had the fish for about a year. He is the last of a group of 5 I purchased for a previous 55 gal tank. The others were part of a die off I had in the smaller tank. When we started this larger tank we kept some water, the remaining fish and the live plants, but replaced the gravel, filter and heater. He has been like this for probably a month now.
I couldn't upload videos, so I've linked them