I think part of the problem with your fish dying is that you tank was far too overstocked. You really need a much bigger tank than that for those fish.
But yes, I would be doing a water change as likely it was ammonia and nitrite that killed your fish. In fact with a tank that small and too many fish you need to be changing the water more likely at least twice a week or even more often than that.
Add water conditioner to get rid of chlorine or chloramines (get one that does both) and that is about all you need.
And if you haven't read the sticky in the newbie forum on cycling fish tanks then I would suggest you do.