August 11, 2013 changed my mind about planaria. I had heard differring stories about harm with shrimp or not. It was around the previous month when out of my 30+ tanks, I got a planaria infection in- out of all tanks- my Nessie tank. I chose to ignore it thinking it was harmless.
On the date mentioned though, I saw this dreaded sight:
Not a dead shrimp, and not a baby, but one of my prized adult shrimp still struggling and attacked by a planaria. Then, other planaria joined the attack. Luckilly I had my camera with me at teh time to record the event for any doubting Thomases. And even then, it won't convince everyone.
So for me, planaria= bad.
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This is a freshwater hydra:
This is a gill fluke:
They are not the same. Hydra have stingers like jellyfish and will sting the pray, and then grab it to their center mouth. They reproduce by budding, so smashing them only spreads them around your tank.