A scientist needs to check several parameters before coming to a conclusion about the amount of water pollution. The scientists need to check the amount of dissolved oxygen in the water, a temperature of the water, the clarity of the water, the PH level of the water and also a number of bacteria present in the water. There may be other criteria’s, but the mentioned ones are enough to gauge the amount of pollution in the water. Scientists often take fish and aquatic plants from the water to be tested to check the amount of pollution indirectly affecting these species.