bengai cardinal unhealthy

[bri.reffin]

my first pair of saltwater fish 2 cardinals. 1 is active when the return pump is on and off and the other seems to be breathing unusual and barley eating. its normal for cardinals to stay in the same spot but something seems wrong, should i quarantine

 

[ggNoRe]

Something I found out the hard way and I really wish was more commonly known is that these fish are VERY aggressive towards each other. I've seen one literally bite another one in half. Also I've put 2 babies in a 20G quarantine and watched one rip all the fins off the other one in just a couple days.