Sounds like this fishman is lieing to you. But, that's not such a bad thing. Turn his lie around on him, and make him eat his words.
If there's a virus going around, tell him that he should re indorse your purchase. He sold you a bad fish, that apparently killed your other fish.
Simply tell him that if this REALLY is the case, he needs to pay for the damages. Otherwise you will find a new LFS to cater too. If he is stubborn, let him know that he is making a poor business decision to take a quick $30-50, rather than the money you'd spend during your future visits.
And of course, whether he gets you some new fish or not, never go back there.
LOL