I did my very first water change today. (was having a service before) and all was going well. I turned on the pump to blow out any water in the pipes. Once I finished and tried to turn on the pump again, it wouldn't work. It turns on and is making all the normal noises but it's not sending the water back to be filtered. My pump is external sitting under my tank.(i'm very new to the hobby so excuse my ignorance). It's connected to a small tank with blue plastic balls. What can I do, I don't want my fish to die.