sadly without proof it is hard to point a finger, a witness or video evidence of that person entering the fish room would be good
if the person refuses to let the police view their fish a search warrant would be needed
then you would need to prove the fish they had was your one, ie unique, special colour, markings etc
the best idea is to ban that person from your property as you believe they were responsible for your missing fish
tell anyone you know who associates with them what you believe as well
you can't accuse someone without proof but you can come to a belief through circumstantial evidence