Not sure if this is normal: when I vacuum the pool, I see the junk that I'm vacuuming up getting sent out in the return lines. Is this an indication that I need new cartridge filters? Or perhaps something else is going on? If I need to consult a pool professional, what do I ask for? (I really don't want/need a service contract, all I'd be looking for is a one-time filter evaluation...). Any suggestions?