Forgot to mention last time a bout the algae tablet that you asked about. No fish food going in a tank wilol hurt any fish, regardless. And many fish will greedily devour any food.
The neons may have carried something. When I have this happen, I like to see if the store's supply is similarly being affected/dying, so I know who to blame.

But seriously, if their neons are also dying, they should credit you for the dead fish. But obviously, don't buy more of those neons.
And BTW, I am concerned more with the death. The photos suggest they are just eating too much. It is the several dying so quick that concerns me. And on the feeding, only feed once a day, and omit the water change day [fish should never be fed before a water change or similar disturbance, and they can wait afterwards for the next day], and only give a small amount. Fish should always appear hungry, that means healthier. This nonsense about several feedings a day for mature fish is just that.
Byron.