Gotta admit, you've piqued my curiosity about your little friend. heheh. A bit (a lot) macabre, but it is interesting when ya think about it. Really not much out there about keeping them alive. I couldn't find any info about anyone doing what you are.

I'm sure there is some medical school or scientist that has studied living specimens, but I can't find them. Hundreds of sites helping to kill them though.
Some minimal info I was able to dig up:
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http://animaldiversity.ummz.umich.edu/site/accounts/information/Diphyllobothrium_latum.html
This parasitic organism feeds on the contents of the host mammals' intestines through absorption. It has no gut or mouth, thus does not contain a complete digestive system. This tapeworm especially depletes the host of vitamin B-12, cleaving and sequestering almost all of the host's B-12. The worm may also interfere with the host's ability to take up the vitamin, thus supplements are needed to combat the deficiency (Roberts and Janovy, 2000; USFDA, 2001).
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http://en.wikipedia.org/wiki/index.html?curid=1034821
The tapeworm can live up to 20 years.
In a small number of cases, this leads to severe vitamin B12 deficiency due to the parasite absorbing 80% or more of the host’s B12 intake
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Most sites just have the same basic info as the wiki page.
Looks like getting some liquid B12 will be your best bet for feeding it. I couldn't find a reference for them absorbing anything else. I'm sure they do, but I can't find what. Maybe get a liquid multi-vitamin too just in case.
Don't cut it in half to get more of them, it won't regenerate a head.
Fenbendazole will wipe out that entire tank (Hydra, Planaria, and the Tapeworm)
If the tank somehow gets cyclops in it, they will eat tapeworm eggs and will then be capable of infecting a human or fish if either were to ingest the cyclops.
Good luck man. Keep us posted.
P.S. After all this reading, I think I'm gonna give myself a tapeworm detox treatment, just in case.

I don't have any symptoms, but most people with them never know it, and i have had sushi once every couple months for the past few years.
