Well, this didn't work out. The fish, once turned on (the switch is quite small), keeps flapping around until the battery runs out or I turn it off, even if the cat isn't playing with it. I used to have a fish that only started flapping when touched—if the cat ignored it, it wouldn't turn on. This one is also quite a bit noisier. What a pity. I won't buy it again, it actually scares the cat more than anything.
At first, my cats were scared because of the movement and the noise. But when it's switched off, they absolutely love it and have been playing like crazy with it for months! You can also hide catnip in the belly really well.
It moves its tail a bit, for about 10 seconds after you touch it. After that, it just lies still. I expected the fish to move so much that it would actually jump around, but that’s definitely not the case.