I could use some advice on this as well. Last night I discovered a host of little critters that look just like that in my newly established 10 gallon high tech experimental (haha) tank. I quickly googled to find out what they are and discovered they are hydra. I have been keeping low tech tanks for close to 15 years and never had them before. I have been doing a ton of research since last night and decided to either add something that will eat them or use some fenbendazole. Problem is, I am not feeding anything to the inhabitants of this tank, so I am not sure why I have so many. I started with only plants for 3 months, and then another month with plants and co2. Bladder shrimp invaded the tank, but I just let then be, thinking they would help the biological filtration process. Since I wasn't adding any fish food I thought their waste would help grow beneficial bacteria. Then string algae bloomed and I couldn't keep up with physical removal. (I am still.working on establishing a good balance of light, nutrients and co2. ) I added 5 amano shrimp and 5 short nosed algae eating shrimp about a month and a half ago. I ordered two Nerite snails, but they've only been in there a week, so they haven't helped a whole lot yet. The shrimp barely put a dent in the algae, so I went to the LFS last week thinking I would get get a juvie true Siamese algae eater temporarily, but the guy talked me into two American Flagfish. They are so shy that I rarely see them, but the few times they are visible I do catch them ripping at the algae. There is still so much string algae though. As I said, I am not feeding the tank yet so that everyone in there will eat the algae. I do weekly 50% water changes to reduce the oragnic waste accumulations from any decaying plant matter and the shrimp/snail and now fish waste. And now I am going to have to battle these hydra! Any ideas on why I have so many hydra? I was originally planning to move a group of Celestial Pearl Danios into this tank. Will they eat the hydra? I've read that they might, but wondered if any one has certain knowledge on this. (I would of course wait until the string algae is under control, and have returned the Flagfish to the store.) I will say though, they are quite fascinating, I almost hate to kill them off.