With so little information it is hard to make a diagnosis.

How many of each were in the tank? Each of the fish mentioned are schooling fish and prefer large numbers. Never do well singly or in pairs.

How big is the tank? How many fish in it, what filtration?

Did you do a water change? What is your water change schedule?

Did you clean the filter out prior to the die off?

Was there any cloudiness to the water?

How long has the tank been established?

What is the pH and hardness?

Planted or not?
Were the fish showing unusual behaviour? Gasping, swimming near the surface, flicking against things? Lethargy?

And many other questions, but these will do to go on with. I know what I am thinking, but without these answers cannot help much further than to say, probably ammonia.