Hey guys, i'm new to these forums so i'm hoping i have posted this question in the appropriate thread. Recently, i noticed the electric heater in my fish tank had a volume of water around it all times which was rising, it made sence to me that the water surrounding the heater would be rising due to density, but the fact that it was visually separated to the colder water puzzled me. It was like the hot water appeared like alcohol, rising with swirls. I asked my friend what he thought it could possibly be, and he said it appeared to have the same characteristics as hot ground does in the desert, shimmering above the surface as the hotter medium rises. But i was not convinced, i was thinking more along the lines that perhaps the hotter water had a different refractive index due to its decreased density, and in relation to the light shining upon it consequently appeared isolated. I'm sure there is many people on here that have a greater idea than me as to why this happens, it would be great if you could help me out. Thanks.