A common problem encountered in salinity-gradient solar ponds is the growth of various types of algae and bacterial populations, which affects the brine clarity and hence reduces thermal performance. Algae and bacterial populations are enhanced by the presence of organic nutrient such as nitrogen and phosphorus. A comprehensive study was undertaken on three salinity-gradient solar ponds in Australia: a 3000 m(2) sodium chloride solar pond at Pyramid Hill in Northern Victoria; a 50 m(2) sodium chloride; and 15 m(2) magnesium chloride solar pond at RMIT University in Bundoora, Victoria. The experimental study involved monitoring the clarity of these three ponds and testing chemical and biological treatment methods to see their effect on the brine transparency. The sources of turbidity and their impacts on clarity and efficiency of salinity-gradient solar ponds are presented in detail in this paper. The initial observation showed that the amount of sunlight is reduced due to the heavy algal growth creating instability in the solar pond as it absorbs light. Two treatment methods were applied to these solar ponds and experiments were conducted to study the turbidity reduction in the solar ponds. In the RMIT magnesium chloride solar pond, diluted hydrochloric acid was injected in the pond to reduce the pH and turbidity levels. Algal blooms were observed and found in the pond where the pH was between 5.5 and 8. It was observed from the experimental study that pH values should be kept below 4.5 to maintain low turbidity and prevent algae growth. The introduction of brine shrimps was also found to be very effective, and economical to control algae, provided the oxygen has not depleted due to advanced heavy algal growth. The investigation concluded that hydrochloric acid could be used initially as a shock treatment to kill all the algae and then brine shrimps could be introduced to control the growth of algal and maintain transparency. This analysis showed that by using a combination of chemical and biological treatment methods, the pond clarity can be maintained and the thermal efficiency of the solar pond can be improved. (C) 2011 Elsevier Ltd. All rights reserved.