Heh, I was kinda freaking out at first, but I think it's more fascination now... Never seen anything like it before!
I figured since they grow really small (like 10 millimeters was it?), their stinger/poison injecting tentacles wouldn't really bother my fish or snail. If my snail eats it, though, then I suppose that's just extra nutrition for her xD I do have water fleas and one of the main sources hydras feed off of are daphnia (which their diet consists of algae and bacteria) so I don't really consider either of the species being in the tank a bad thing...
I feed my snail algae wafers so even though my algae problem has gone down (there's still a small bit here and there, but overall there's not really much in there for it to count), it won't dismiss small particles of algae stuff that gets left behind- I have a mix of gravel and sand/plant substrate so I'm not able to siphon out EVERYthing, but again my water quality hasn't been showing signs of ammonia or anything of the sort so any debree that gets left behind isn't really noticable for my liquid tests... Even so, I'm not going to lean towards over feeding because I've actually been feeding my snail less algae wafers since controlling her feeding habits will help to slow her shell growth- and I'm trying to make sure she gets good even growth so her shell will stay healthy/tough on all sides. My betta, I feed twice a day, but it's usually 3-4 pieces (mostly three) each time. There are occasional moments when a piece or two hit the floor of the tank, but it's very seldom. My betta never seems to notice the water fleas, at least from what I've observed since I've noticed them in my tank (which has been about 6 months)... So I suppose having the hydras will help lessen the number of daphnia for me.
That aside, apparently seeing creatures like daphnia and hydras show good water quality and that the ecosystem in your tank is healthy- at least that's what all the research I've come across about the two have shown me, but that's just my research... It seems like unless I were to take the tank completely apart and start from scratch, I won't ever really be rid of either since daphnia are so tiny I can only get rid of the ones that get siphoned out of the tube. And apparently hydras can break into segments and grow just like any worm would. Not to mention hydras don't age, so they're technically immortal xD I think seeing as how their threat in the tank is little to none, I'll keep a few in there so I can observe... I don't know many people- well... any that have hydras in their tanks, so I think it'd be an interesting thing to keep for the time being.... If things start to look like they're going ary, however, I'll definitely start working towards getting rid of them.