Can't figure out what's wrong with lethargic, drifting danios


What is the water volume of the tank? 20 gal
How long has the tank been running? 3 months
Does it have a filter? yes
Does it have a heater? no
What is the water temperature? mid 70s during the day (80 on a hot day) and ~65-75 at night.
What is the entire stocking of this tank? (Please list all fish and inverts.) 5 longfin zebra danios, 1 longfin leopard danio, 2 golden danios. Bladder snails. Floating anacharis, dwarf lilies, and an aponogeton.

How often do you change the water? Usually every 2 weeks. Lately, every few days.
How much of the water do you change? 20-60%
What do you use to treat your water? Fritz guard
Do you vacuum the substrate or just the water? Substrate

*Parameters - Very Important
Did you cycle your tank before adding fish? No, I did a fish-in cycle with frequent water changes
What do you use to test the water? API master test kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0
Nitrite: 0
Nitrate: 5
pH: 8.0? (I have a hard time reading the color chart on this one, but it's the same as it's always been since I set up the tank)

How often do you feed your fish? Usually 1-2x a day, but I tried fasting the fish for 1-3 days at a time
How much do you feed your fish? We used to feed them as much as they'd eat, but I've dialed back to one small pinch at a time.
What brand of food do you feed your fish?
  • Daily: My LFS's tropical flakes: 45% protein, 4.2% fiber, 4.1% fat, 8.2% moisture (ingredients: salmon meal, wheat flour, soy seal flour, brewer's yeast, corn starch, plankton, krill, kelp meal, brine shrimp, fish oil, lecithin, spirulina, vitamins A D-3 and B-12, Choline HCL, Biotin, Vitamin C, methionine).
  • 1-3x per week: Gamma frozen Tropical Quintet (they mostly like the brine shrimp and blood worms)
Do you feed frozen or freeze-dried foods? Frozen

Illness & Symptoms
How long have you had this fish? 2-3 months.
How long ago did you first notice these symptoms? ~2-2.5weeks ago, but some fish may have had issues for longer
In a few words, can you explain the symptoms?
My fish have become lethargic and often stay still; some sink when they aren't moving. Sometimes fish won't come up for food. Tails droop or lift when they aren't moving. Some fish are seen breathing heavily, kind of stuttering to stay in place, or fluttering their fins really hard. One fish is noticeably worse off than the others. Possibly some gil irritation on some fish, as well as what looks like anal inflammation noticed just today on a few of them.
Two deaths since setting up the tank:
  • One leopard danio found dead on the gravel the day after it was added. No signs of disease, but it was pretty sad-looking at the store so I wasn't surprised
  • Just over 2 weeks ago, another of the leopard danios found dead on the gravel in the morning with no outward signs of disease. It had always been skinny but it had gained some weight since I brought it home.
Have you started any treatment for the illness?
  • Put a small amount of salt in the tank a couple weeks ago (worried about the plants) -- probably all gone now from water changes
  • Gave the worst affected fish a salt bath about a week ago. Slight improvement the next day?
  • Tried fasting + peas once. Peas without fasting once. Fish don't eat the peas when I'm looking...not sure if they ate any.
  • 24 hour treatment of levamisol on Friday per package directions (dosed directly into the water)
Was your fish physically ill or injured upon purchase?
  • The leopard danios (3, now down to 1) were thin and not very lively when I bought them. Often stayed near the bottom of the tank, but they still came up to eat and would chase the females around. All other fish were healthy and active.
How has its behavior and appearance changed, if at all?
  • Some fish are more active than others, but overall they rarely chase one another at all now. Fish have pretty much all slowed down and they now hang out in a loose group in an open area near the bottom of the tank pretty much all day. Haven't seen any breeding behaviors in a while. It seems like the ones that were small when I got them have stopped growing, as well. Much less activity during feeding time, though it seems to be improving? My big females were getting thin but I think they've started gaining weight back. My former "queen of the tank" is the worst. She's wobbly and jerky with a droopy tail and seems to be sinking or gasping half the time. Today I watched the tank for ~20 minutes and noticed that every so often she'll zoom around briefly. A week ago (before the salt dip) she wasn't eating, but recently she seems very interested in food and she's swimming better, so I'm hoping she'll pull through since she's my favorite :(

Explain your emergency situation in detail. (Please give a clear explanation of what is going on, include details from the beginning of the illness leading up to now)

My tank of danios is going downhill slowly (see above) and I think it might be mycobacteria and bad danio genetics? I don't see any outward signs of infection or infestation despite squinting my hardest during every work break. They still look shimmery, sleek, and bright. Tank parameters have been stable for months, but at one point I did let the uneaten food and plant debris build up in the gravel. The plants were sucking up so much nitrates that my test kits had me thinking I could keep skipping water changes. At any rate, it's vacuumed really well now!

The 5 longfin danios I cycled the tank with were awesome, so when the cycle finished we added healthy-looking golden danios and skinny, droopy leopard danios to up their school size. I thought the leopards looked suspicious, but stupidly put them in without quarantining them, thinking they were probably just malnourished and that I could treat a disease outbreak if I had to...after all, replacement danios would be cheaper than a quarantine tank and filter, right? I was convinced that danios were bulletproof, too.

One of the leopards that was always skinny and slow -- but nonetheless a participating member of the group -- died in the first week of June. While it may have started before then, I've noticed that my other fish have been succumbing in various degrees to lethargy, drooping, and moderate weight loss since then. Things looks to be slightly improving since I cleaned the gravel better, but at this point all my fish are noticeably subdued. Only one of them seems to be in really bad shape but she keeps showing signs of improvement, so I haven't put her down yet despite maybe a week and a half of being really worried about her.

I suspect that I brought in a disease with the leopard danios and it's only started affecting the rest of the tank this month. Ironically, the only remaining leopard danio is now one of the healthiest-seeming fish.

What I've tried treating:
I did a 3 day fast last week and then frozen peas with no takers. After the fast, I gave some frozen foods that the more active fish seemed to like. I did a lot of vacuuming after the frozen foods and peas each time because of the mess, and the fish seem to perk up after each water change and eat better.

Internal parasites?
I wasn't able to get to the LFS the past two weekends and Petco doesn't stock any good meds, so I ordered levamisole online which just arrived last week. Last Friday, I dosed the tank per package directions, changed the water 24 hours later, then had to leave for the rest of the weekend. Upon returning, the fish seem the same. However, I did notice today that a few have pinkish anuses and possibly some missing or inflamed gills? Hard to tell since they've always had somewhat transparent gill plates. I looked all over for signs of parasite poop and found nothing, but I also haven't seen any of them pooping ever. From observing what I suck up with the gravel vac, their poop is normal little brown turds.

What next?
I'd like to try an antibiotic before I start euthanizing.
Suggestions? It's okay if the antibiotic will kill my plants, because if these fish all die I'm going to nuke the tank anyway. :'(
Video of my sickest fish, Becky:

Becky is circled in the full tank picture


  • whole tank shot 1.jpg
    whole tank shot 1.jpg
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Hi, symptoms like that could point to oxygen deprivation, danios prefer cooler temperatures & the warmer the water in their tank the less dissolved oxygen it can hold, what's the surface agitation like in your tank, from the photo it looks pretty still over most of the water surface area & that's where most of the oxygen transfer happens.
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Thanks for your help! We did have a small heat wave around the time the issues with the whole tank started (tank was reading 78-80). It's been cool the last week or so, with the tank going from about 70-74 throughout the day.

The surface only has bubbles above the sponge filter. The bubbles usually pop on the anacharis and don't make it across the tank.

I'll get an airstone from Petco to try and I'll order some fans online as a stopgap until my air-conditioned office is ready.
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Update: the bubble bar from Petco is pretty crappy, so I'm going to get a new one and a more powerful pump at the LFS this weekend.

But the fish do seem overall more lively this morning! I'm hopeful they'll keep improving.

thanks you guys :)
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