Good idea on paper, but realistically, what would stop the owner from getting more...
Best way to probably convince them this is a bad idea is to use reverse psychology on them...
ask them if they are getting it to bring good fortune to the restaurant, and if they say yes, then say wouldn't one or more of them dying within a few weeks be considered bad luck? if he again says yes, then you could go on to explain how this is a very likely situation with 4 large fish in a small tank and go from there...
Being half Chinese i know all about how people are when it comes to superstitions in Asia... and the only way to convince them is to explain how their idea is flawed and how it could backfire...