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Forktail Rainbowfish Swimming in Place at the Bottom of Tank


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I've had these rainbowfish for ~4 months now, and they've been bulletproof until today. I noticed one of my females resting near the bottom of the tank and sort of swimming in place. She looks normal, except her fins are tucked and there's something dangling just in front of her anal fin.


So far, she's done this twice today. The first time lasted ~30 minutes before she swam up and was back to normal for a few hours. As she was swimming up she was flaring her fins a bit. Second time was about an hour ago and she just recently went back to swimming normally.

I have a hard time believing this is normal behavior, but nothing in the tank has changed for weeks. Tank parameters are 0/0/20 8.0pH 78F in a 20 gallon long with several other forktail rainbows, none of whom are acting strangely.

They're fed a varied diet of frozen worms/daphnia, live baby brine shrimp, BugBites flake and VibraBites.

Could this be related to spawning behavior? I've found a few eggs in the sponge filter, so I know they've spawned in the past. I'm guessing parasite, just based on the dangly bit attached to her body, but I figured I would throw it to people with more expertise diagnosing things like this.

Thanks for any help with a diagnosis!




Edited by Schwack
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I've had someone suggest camallanus worms, and that seems to line up with what I'm experiencing. Prior to this, I lost 3 spotted rainbowfish over the course of 4 months. They were all 6-8 months in my care, so I chalked it up to age. Everything looked normal about their bodies and I know they're a short lived fish. Since everyone else was behaving normally age seemed like a reasonable cause of death.

Looking back, and through posts by other who've battled camallanus, that seems like a strong possibility. There is a small, red thread hanging from her vent. It was also pointed out that she has a red spot on her abdomen, which shouldn't be there. Based on her behavior this morning, it's far too late to save her, but I'd like to save her tankmates.

I ordered some levamisole from select aquatics. I'll dose the tank my rainbows are in, and likely at least one other. I transferred some cory cats out of my rainbowfish tank and they almost certainly carried it to their new tank. I've got a small tank with CPD and fry that has shared tools/equipment, and I'm not sure if I should treat them as well. Anyone who's experience a camallanus outbreak able to weigh in?

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I’ve not had to treat internal parasites but I would not recommend preemptively treating the CPD tank, wait until you see something come up.

This way, you can avoid the possibility of parasites being resistant to the medication you just used, which would leave you back at square 1. 

I would recommend having different equipment for each tank, it can save any future outbreaks spreading😉

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Sad, but informative update. As expected, she didn't make it. I had been considering humanely euthanizing her, but she beat me to it. I must've found her shortly after she passed, only my jerk of a pleco had found her and was trying to make her into dinner.

The upside is that I was able to perform a bit of a fish autopsy. As grim as it was, I feel like it's something I'll be doing any time I run into a mystery death in the future. Had I opened up the first fish I lost, I feel confident I wouldn't have lost 3 more. Long story short, she was full of worms. I'm assuming these are camallanus worms, but hopefully someone can help with an ID.

Warning: Worm pictures in the link below. I don't want to attach them directly to this post since some folks get squeamish around parasites (myself included. Extracting these was not a good time.)




They were still drawing blood and moving around when I removed them, and I'm guessing this is just the tip of the wormberg.

So, can anyone positively ID these as camallanus worms? I've got an order of levamisole on the way, but it won't arrive until Thursday. I called a bunch of farm and ranch stores nearby, and none of them carry it as a goat/cattle dewormer, it's all fenbendazole.

I contacted a fish store in Portland, and over the phone they said they carry one product with levamisole, so if I can get a positive ID I'll be cruising up there tomorrow. I can see my other female forktail is behaving erratically and I'm afraid waiting until Thursday will mean she's gone too.

Anyway, I wanted to post an update in case someone down the road runs into a similar bunch of mystery deaths. Even if their bodies look totally normal on the outside, it may be worth your time to glance at their internals when you have the chance.

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Hopefully this doesn't count as a bump, but I figured I'd document my experience dealing with these stupid worms in case someone ends up searching the forums for them in the future.

I haven't lost any fish or inverts since I dosed with levamisol on February 2nd (200mg). Per the instructions from select aquatics I did a 25% water change 24 hours later and followed up with a second dose. There was quite a bit of fish poop after a day or so and I noticed one fish appeared to be showing signs of popeye. Water tests showed .5ppm ammonia 0 no2/5 no3. I was a bit concerned, but I figured the raised ammonia levels were due to rotting organic material/worms the fish had expelled. I did an extra thorough vacuuming of the substrate, got things nice and clean and waited another 24 hours. There were still detectable levels of ammonia and my nitrates were sitting very low.

Checking the other tank I dosed with levamisol and it was also showing detectable levels of ammonia. It appears as though the levamisol crashed the nitrogen cycle in both tanks. Super confusing. It left all my inverts alone but looks like it wiped out my bio filter.

I've since moved seasoned filters and substrate from a tank I've been keeping ready for fry into each treated tank. Since the stock levels are very low, I'm hoping this will provide enough biofilter until the tanks can recover... unless of course the residual levamisol disrupts the bacteria on the new media.

Anyone else who has dosed levamisole run into this problem? If it kills the bacteria on the seasoned media I added, I'm not really sure what to do. I'm planning to treat the fish with popeye with some maracyn in a QT tank, but these cascading problems are killing my motivation to keep these tanks going.

Edited by Schwack
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