Summarize why the fish heart and circulatory system are considered important evolutionary changes

Fish has a singular circulatory system, means the blood carrying oxygen from pulmonary circuit will be dumped into the same chamber as the blood carrying wastes and CO2 in the heart, because the heart has only one chamber. 

amphibians has a 3 chamber heart, but still not a complete separated pulmonary circulation. However, it does have 2 loops at this stage. Of course birds and mammal also has this feature. So the distinction will be between fish and amphibian, which is the evolutionary double loop pathway.