What Happened?!

[NeonSagaris]

I was out for a day and a night a few days ago, and I came back yesterday to find two dead Endlers and the rest of my fish losing their colours! I was so sad to see the deaths.... 

 But now, the three remaining Endlers, the Gouramis, Molly, Otos and Shrimp are absolutely fine and swimming around very happily. I was only gone one day and this happens! But at least they're happy again now and have their colours back.
 
I told my brother to feed them a small pinch of the Tropical fish food in the morning and evening, and accidentally, he fed them Goldfish food, but luckily small amounts. Could this be why my fish were so unhappy for that one day? The tank was due for a water and filter change which I have now done, but I doubt that's the cause.
 
I am getting three more Endlers on Friday.
 
What could be the cause of this sudden unhappiness? And, what am I supposed to do when I go away for longer times?
 

 

[techen]

He could have fed too much and caused a ammonia spike which would have led to a sudden death. But am not entirely sure what's in goldfish food? I doubt it's much different.

 

[NeonSagaris]

Probably not, but it's weird how my fish could fall ill and a few dead so quickly with me not being there...