Small Anemone Looking Things in FW

I have a fresh water tank that has several tiny crystalline anemone looking things growing around stems of plants. They are spherical and about the size of a wooden match head and when I poke a wooden kabob skewer (I use them for some planting stuff) they retract their tentacles.
Any idea what these might be? Can they cause problems for my fish?
Thanks!

Those are Hydra, which are basically like freshwater anemones. They will sting and kill fish fry, but I don't think they would take down larger fish unless maybe in extreme numbers.

Those are Hydra, which are basically like freshwater anemones. They will sting and kill fish fry, but I don't think they would take down larger fish unless maybe in extreme numbers.

I have seen Hydra pop up in some of my native tanks, but these look different. Hydra has a stem that attaches to the glass or plants, the things I am seeing are just a ball of outstretched tentacles around plant stems. They have many more tentacles than the Hydra I have seen- maybe a hundred or so. They are popping up all around this one tank I have.
Tried to get a pic, but no luck with my camera.

Have you ever put fed tubifex, blackworms or white worms to your fish?

Have you ever put fed tubifex, blackworms or white worms to your fish?

Yes. Blackworms.

Edited by Elijah, 03 June 2010 - 06:15 PM.

Its possible what you are seeing are small little colonies or blackworms. To make a short story long...I had a 28 gal bowfront set-up a little while back. I put peat, clayey soil from my back yard, then balck flourite sand, then a dark coarse sand on top - planted it. Looked nice. I forgot about the live earthworms in the soil from my backyard. I took the fish out a couple months later because the water got soooo cloudy. Fast forward 8 months...the plants were doing good but most of the clay had been pumped on top of the sand by the worms and I had a really cool little ecosystem.

The cherry shrimp had exploded hundreds of them...and the blackworms I had originally seeded had taken off as well. In addition to a fairly even carpet of wavy little worms ends protruding from the mud there would often be a large group (10-20) sticking out of a single hole left from an earthworm. They would stickout a very short amount. The entire tank bottom was a soft mud from the clay. If your tank is planted with a more coarse substrate the worms may be sharing the softer areas at the base of plants.

Speaking of colonies, other possibilities include colonies of large protozoans such as Stentor or Vorticella, as well as sessile rotifers. Each "tentacle" in this case would be an animal.

Speaking of colonies, other possibilities include colonies of large protozoans such as Stentor or Vorticella, as well as sessile rotifers. Each "tentacle" in this case would be an animal.

Ah, this seems more like what I have. Vorticella is most likely what it is.
I could not find specific info about this protozoa on fish. I am wondering if these can be harmful to my fish?

Nah, they strictly feed on smaller organisms and produce no toxins (as far as I know). Many fish like to graze on these little guys.

Nah, they strictly feed on smaller organisms and produce no toxins (as far as I know). Many fish like to graze on these little guys.

Sweet. Thanks for the info!