Sick Fish?????anyone....

I have a 46 gal bowfront cichlid setup. The setup has moon rock and eco-complete gravel. I have about 15 african cichlids(mostly Malawi). They hardley ever fight and they always eat. Recently I switched over to a Rena canister filter. I'm not sure if my incident is tied to that. I noticed the other day that about half of the fish maybe more are moving their gills/mouths rapidly. They sometimes scrape against objects in the tank. The scraping is inconsistent. They still eat. They still have great color and swim around like normal. I'm not sure if maybe I just never noticed it before or if its new behavior. Input anyone......Please

Thanks in advance

Greg