The present invention relates to anionic copolymers, which are suitable as scaling inhibitors in water bearing systems, in particular as inhibitors for the formation of antimony(III) sulfide-containing scale. The present invention also relates to a method for reducing scaling in water bearing systems, in particular in water loops of geothermal power plants, in which the formation of sulfide-containing scale and especially the formation of antimony(III) sulfide-containing scale may occur. The anionic copolymers have the following characteristics: 1) The copolymers comprise a. polymerized units of a monomer M1, which is a primary amide of a monoethylenically unsaturated monocarboxylic acid having 3 to 6 C atoms and b. polymerized units of a monoethylenically unsaturated monomer M2, which is selected from monoethylenically unsaturated monomer having a sulfonate group and or a sulfate group and monoethylenically unsaturated monomer having a carboxyl group and mixtures thereof; 2) The copolymers have an anionic charge density in the range from 0.5 to 6.0 mol/kg, 3) The sodium salts of the copolymers have a weight average molecular weight Mw of at least 10000 g/mol, as determined by gel permeation chromatography.