Whats up with my cleaner shrimp?

[Nano]

So I was told I used my voltage meter incorrectly. So I put the positive in the ground of the plug outlet, and the negative in the tank. The needle moved, I will put a picture below, with a black line pointing to the exact location of where the needle landed. Now, I am a noob to all of this, so I don't know what it means lol, I just know the needle moved. any help on how to solve the problem, or what I did wrong or right is greatly appreciated!!