I will add in my $0.02 here as I have been around wellwater my entire life. There are two different reasons that will make the water smell like sulfur. The first, is what most people have already explained, that there is naturally occuring sulfur in the well and it just comes with the water. This is harmless and probably wouldn't bother thei fish at all. The second kind comes from a bacteria that harbors in wells and hot water heaters, this is an unknown as to whether it will harm the fish or not. I would recommend getting your water tested, just to be sure.