First I wouldn't plug it in before it fills with water. I did that to some old filters and its not good for the impeller. You should first prime the filter, which means to fill it with water from the tank or if necessary another source. My ehiem eccos have a handle that allow me to prime the filter without having to suck on the tubing or anything. I don't know about the classic, so I would check the user manual or maybe someone with a classic can answer this question.