I am fairly sure it isn't hydra. As I tested another shrimp in the net just within the water and it was playing up. But I know what you mean by appearing to be paralysed. I am not sure that whatever it is isn't playing the corys up. They are not scratching any more, but they are still really docile not happy foraging like when I first got them. I am going to tear my hair out at this rate lol I started the tank as i wanted to keep shrimp more than anything. p.s. My water is uber uber uber clear now lol suppose that's one good thing. Once I get another bucket free i will start testing with substrate but i cant for the life of me think how an inert substrate can cause problems like this.