I have an ASM 1 with a Mag 5 pump. Recently I've had to prime the pump( cover the inlet with my hand) to get it to suck air. After a water change this evening, the skimmer isn't working in the sump. I can pull it out and put it in a bucket and sometimes it runs, but it won't do anything in the sump. Should I try vinegar? I'm kinda stumped. Any ideas?