As long as the ammonia and nitrite readings are at 0ppm, it is not "overloaded". You need to double check on these readings to make sure. You have a filtration problem only (if) there is indeed ammonia and nitrite, even trace of it. Otherwise there is no need to look that way for better filtration system since it is not the problem. It is a good idea to use a liquid test kit. It is more accurate.
I still think the problem could be the shocking effects from the water change. Either that, or the fish are actually of low quality. Some fish are bred from low quality fish farms with weaker immune system. They are more likely to die even when the healthy fish are doing perfectly fine in the same fish tank.
By the way, double check the water temperature to make sure it is ideal and stable. It is also one of the first things I check if there is a problem. Followed by testing the water for ammonia and nitrite. We have to rule out the possible problem one by one.
