I have a new tank and a fish died. Why?

This is day 4 of my new tropical fish adventure. I put 5 fish in 2 days ago and I found one dead this evening. The water tests were ok. Maybe the nitrite level was a bit high but it was boarderline/ambiguous. I did a partial water exchange. I've followed all the instructions exactly. what could it be? I also noticed his tail was missing. Could he have been attacked by the other fish? (They are not nippy fish - all non aggressive) Or would the other fish have had a go after he died?