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Do water problems affect larger fish earlier than smaller fish?
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Yes, ammonia burns the gills, the bigger fish use more oxygen making them more likely to succumb first. But even a non-fatal level of ammonia or nitrite can hurt your fish in the long run, so its good to keep them down with water changes.
I agree you tank is either still cycling or in a mini-cycle from the addition of new fish. The ammonia going down to 0 is normal and desirable, next you watch for the nitrite to go down.