Yup i can always tell how the tank is doing based of my congis.I remember J jaws7777 saying that his Congis are the first to show physical indications of any water quality issues that he has. Have you checked your nitrates lately?
Others will recommend feeding with lights out for picky polys. Have you tried that? They don't look overly skinny to me. Is there a chance that they're just slowing down in growth because they're at a length where they do that? How big are they?
I'm sure others will chime in. I don't have any LJ experience.
Could be the new additions or some other fish causing some sort of stress at lights out.
All my polys slow down when they hit 12ish inches tho