So I went into the aquarium store to ask about the worms. When I walked in, I said to the folks behind the counter, "I have some strange worms in my tank. Just a sec, let me get out the pictures."
"Finally!" they exclaimed. "Somebody brings pictures!"
From which I concluded that weird worms in tanks are not something weird at all. The aquarium folks first suggested that it was planaria, flatworms, but when I said that I hadn’t seen any arrowhead-shaped heads, they jumped phylum and said they were probably leeches. But not necessarily bloodsucking leeches. I spent many summers in Northern Minnesota, so I am conversant in the difference between the bloodsucking leeches and the two-foot long sludge-eating leeches.
They agreed with KaosDevice’s assessment that the worms probably came in on the blackworms. To quote: "Blackworms are grown in some pretty disgusting places and a lot of things get in there." This isn’t terribly comforting considering that the first time we bought the blackworms they were sitting in a dish of water right next to the register and the counter-girl was inviting people to poke them.
Is this a good time to talk about my history with leeches? I had a pet leech as a kid. I lived on the Wisconsin River at the time, and I kept a five gallon tank where I threw buckets of sludge and algae that I found on the river. There’s some amazing things in that river, flatworms and bugs, and things that twitch and wriggle and slide.
When I found the leech it was stuck to the inside of an old clam shell. The leech had a pattern of red and blue spots down its back that went well with the opalescent sheen of the shell. So I scraped him/her into a bucket and took it home. It looked kinda like this:
Only in color of course.
Well, she disappeared for a month, and when she reappeared, she had this weird fur on her belly. Only on closer inspection it was actually thousands of baby leeches waving their mouthparts in the water.
So I freaked and dumped about four pounds of salt in the tank.
Then in college I used the martyrdom of the leeches as the basis for a cult. But that’s an entirely different story.
Just for fun, here’s a video of a giant leech eating an earthworm.