Camallanus
If you have this round worm, your in for a rough ride. Known as a fish keepers nightmare, I would have to agree from my own personal experience. My symptoms include sudden deaths, fish becoming skinny for months before I saw this worm sticking out my fishes bottoms. I haven't seen a worming treatment in my local fish store for treating fish (and tried many fish wormers with no success, mind you this was about 3 years ago) and there for conducted my own research and experiments. I read that levamisole hydrochloride was effective on getting rid of the worm by paralysing it. I also discovered that some bird wormers where ideal to treat the fish. I would suggest speaking to your vet (yes they will look at your fish, and if your luckier than myself, you'll find an expert that can sort your fish for you).
If you fail to get a vet, I would recommend trying treatment by buying Harkers harka-verm sold for birds, as it has Levamisole at a 8% concentration (kindly pointed out to me by betta fish as a better birdwormer than my original bird wormer used). You would need to add 0.04ml per litre of Harkers harka-verm(differnt amount for different wormers) , so for a 20 litre tank = 0.8ml, just work out how many litres are in you tank and multiply by 0.04 for correct dosage. keep an eye on the fish during treatment and make sure you do a thorough gravel vacum on the next water change, usually in a week to pick up any parasites, the fish should then be treated but if you don't think they are all gone you can wait a few more days and add another dose.
Levamisole is light sensitive, so leave your lights off as the light destroys the chemical and will prevent the treatment from working.
betta fish also produced this link on
levamisole-hydrochloridewhich I used when treating my fish at the time of infection