I'm really sorry for you loss, I'm sorry I didn't say so sooner!
I'd say that you should be careful about whether the others might have the same thing. Keep the tank clean - clean fresh water and pristine water quality is the best medicine possible, and monitor the other fish for any signs of illness. If you see anything wrong, take photos or even better, a video, and post here for more advice
There's every chance that the rest are okay, but if you take extra care of the tank right now, it increases the fishes chances of fighting off any potential pathogens.
Just nicked this advice from one of
@Colin_T 's previous postings, so all credit goes to Colin! He's the disease expert, and gives this advice to follow after losing a fish, especially when the cause is unknown;
"
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the remaining fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens."