HELP: when to add new fish

I have had my tank for about a fortnight and everything is working well.
But I am wondering when to buy my next batch of fish. About 5 days ago I bought 6 harlequins one of them died on the first day; it was very pale and wasn't shoaling with the other 5, the rest of my fish have been fine.
I am wondering whether I can now go and buy a new addition of fish the tank. Thoughts?

 

Have you cycled the tank?
What size is the tank?
What other fish do you have in there?

 

Have you cycled the tank?
What size is the tank?
What other fish do you have in there?

Yes
43l
6 harlequins

 

I wouldn't add anything else. If it's a 10 gallon.

Did you do a fish in or fish less cycle?

 

I wouldn't add anything else. If it's a 10 gallon.

Did you do a fish in or fish less cycle?

Fish less

 

What did you use to cycle the tank? Honestly the harlequin rasporas are not a good choice if you have a 10 gallon. They need 20 gallons at least as they are schooling fish.