I have a literal forest of Hydras growing on my cave for my loaches. The little things are tiny, less than a cm in length but there must be over 200 of them on the cave and it is only 4 inches lng and 4 inches across. The cave isn;t the only place as I have seen them on every plant and every side of the tank including the filter but the cave is the worst. I thought about taking it out but they shouldn't be a threat to my neons and I am not breeding any small fish so I will let them eat the other critters and when the population drops enough the hydra numbers will drop off. I hope so anyway.
From what I understand about them, they are predators only meaning theyy need a live food source. I couldnt think of what they might be eating until I saw one with a white belly. They are eating the copopods/fleas that were infesting my tank. The copo/fleas are less in number but there are still plenty to feed quite a few hydras so I guess they will be aorund for a long time.
Oh well, another one of those indicator things that says the tank is healthy and clean otherwise I would have them.
Just remember if you ever see them it is a good sign of a healthy clean tank and not something to worry about. Chances are you have them anyway and have never seen them.
The picture isn't very clear but you can see the distinct color variation on the green cave in the picture. I wish I could get a good picture of them because I can see a couple of them that have at least 3 stalks with tentacles.