The thing that I don't understand was that I had gone in to buy fish, the sale was guaranteed. Instead of buying the 4 danios I'd intended, I bought 4 Tiger Barbs at the same price as Danios.
Perhaps he thinks I'll kill them all and go back for more and therefore spend more money.
The remaining 3 Tiger Barbs seem to keep falling out with each other - one of them is patrolling the tank and keeps chasing the other two. Next minute they're all friends again zipping round the tank together ... is this normal behaviour?!