Killing Blackfish

One of my blackfish is growing a hump on its back. This is called 'Fish TB'. I believe at some point in time the fish will die from it. And unfortunately it's contageous. This terrifies me. I do not want to have to kill one of my fish. But I do not want all of them to die. They sit right beside me every day as I read on the couch. They stick their little faces right up to the glass and watch me. Then they will look up and beg for little worms. So I drop some in. And now one of them is beginning to die. Ugh, it breaks my heart. I'm terrible at getting attached to things. I cry when my garden dies in the fall. And so now I have to make the sacrifice of one of my dearly treasured fish to spare the life of the others. Why must my weak mind be put through such drama? It's like killing one of your children. Not literally of course, but for a teenager like myself it seems the same...