Just about what Stella said...
inanga have a vertical-ish tail where as smelt have a pronounced v shape.
Of the first 1/2 dozen smelt that went in to my tank I think about 5 died with in 48 hours, of the next 2 lots of a ~1/2 dozen I think I lost only one. Big tank and cold water helps a lot I feel.
Went on a Kokopu hunt last night on only saw a few bullies, smelt, trout and 2 eels in a stream that I thought would be ideal very little in native or exotic fish life.