Thanks for the comments. Kulkwitzer lake is a very nice and clean lake, one of the best freshwater diving spot in Europe (It's usually selected to the top10 in scuba diving magazines) and I'm lucky enough to live nearby. But yesterday the visibility was not so good. We had some rain before and it was quite windy.
I tried my old minolta 100mm macro first time in a lake. Although the visibility was good it's not comparable to tropical sea conditions and usually shorter focal length lenses are recommended. Nevertheless I gave a try. It worked, when I managed to get close enough to the fish, but the 50mm macro or a wide angle lens is better most of the time. We found a giant school of fish (hundreds of them) and my lens was pretty useless. The other problem was that my copy is showing its age and many times it had problem with autofocusing, especially driving from macro range back to far distance.
Some previous photos from two years ago with the sigma 15mm fisheye lens:
Marcell