They would not be showing aggression at this point in time, but I have a feeling there was aggression at some point if the back fin is a tad torn. You need a larger tank for the trigger. By chance, is your NO2
(nitrite) measurement in ppm
? Just curious since NO2
, in ppm
, does not pose a threat to marine fish unless within the 100's+ and even invertebrate larvaes are able to withstand 10's of it before succumbing. If anything you can perform a 25% water change, wait a couple hours, and then another 25% water change. Placing some cover around the animal may also help...can you get a pic of the animal?