OOhhh I got a snail!
I went deep into the forest and plucked a moss-covered rock out of a creek bed and put it in the tank and now I have a snail. I don’t actually think he was on the rock though. It isn’t that big and I did rinse it off and pick off small twigs and such. I think I would have seen him. I’m guessing that he’s been in there all along and I’m just now seeing him.
His name is Shelly.
The plants got too unruly so I had to trim them down a bit and now the Hydra is gone. I didn’t trim the leaf he was on but I guess I disturbed him enough to dislodge him.