I'm having a simmilar problem with my 28g nano cube, one thing that helped was cleaning the pumps. That's something I forget to clean on a regular basis and you will be suprised how much crap get stuck on it. Those all in ones are tricky as everything is tucked away and you can forget about the small things. If you haven't cleaned them in awhile give that a try. It won't fix your problem but it won't hurt.
Sorry this isn't a complete fix
