Air pollution and water pollution are more often problems than problems.

Air pollution and water pollution are more often regional problems than global problems. Water pollution involves the contamination of water bodies such as rivers, lakes, oceans, and groundwater; usually caused by human activities. Air pollution is the release of pollutants into the air; harmful or excessive quantities of substances such as gases, biological molecules, and particulates. 

You should never redesign your experiment. try and see what in your experiment you made a mistake on or try and test your experimemt with somebody else. see if they get the right results.