I find these guys in my Discus tank and my angel tank occasionally.

They live WAAAY down deep in the gravel & don't really seem to hurt anything. I've never seen one attached to a fish or anything like that. I mostly find them when gravel vaccuming.

They are probably 1" long, very thin & fleshy - the exact color of a snail. They swim like an aquatic snake does and I've noticed they have the ability to latch on to things with one end of their body. So in other words, they have some sort of suction cup at one end.

I've had no sickness in these tanks, so I'm only mildly concerned.

So what is it? Is it harmful? Should I be taking them out & "offing" them when I find them as I've been doing?

Here are two pics. Remember...these were taken on a cement driveway. They are only 1" long and normally look slimier and wetter than this...

Do you feed your fish live worms or live food?

They get frozen bloodworms & frozen spirulina/brine shrimp.

Waaay back there had been live BBS in the tank, too. But months and months ago.

Updated:
My August Aquarium Fish Magazine came & they had an article in it about "unexpected guests" in your aquarium. It covered everything from snails to insect to worms and duckweed.

In the article they described a small (0.75" max) harmless type of leech that normally doesn't bother fish because it hides in the substrate - but often becomes fish food if it leaves the substrate.

Does that sound like it may be possibly what I'm seeing? It would explain the swimming motion and the suction cup at one end.

I would have said 'Worm' from the pics. But 'Leech' does fit your description.

It's certainly big enough. I've had these tiny little white wormy things, that walk from end to end on the glass. Only as thick as a cotton thread tho'.

I'd love to find a Hydra !

The gravel gets a good cleaning two or three times a week. Although I must admit that I probably overfeed & spoil the discus. It's easy to do when you know you're water changing all the time. They're fun to watch eat -- guilty.

only just noticed this thread hence the late response. i am 99.9% certain it's a leach (as ur magazine describes) and damn it makes my skin crawl just looking at it.

Sure looks like a leach, but i would imagine if your discus caught eye to it, it would be a quick snack.
The little white thread one sounds like Planaria, wich is a harmless little flatworm, that hitchhikes on plants and driftwood and the such, and also makes tastey little snacks for your fishies.

I'm 99% sure they're leeches...

And I've seen one sucked up by a discus since. Free food is free food.