In certain water-cooling systems, continuous low-level application of oxidizing antimicrobials will not provide year-round control of algal biofilms in cooling towers. Therefore, these treatment s are often supplemented with periodic additions of non-oxidizing, antimicrobials, especially to control seasonal algal biofilms. This report describes a solid combination product that will deliver both oxidizing halogen (BCDMH: bromochlorodimethylhydantonin) and algicide (Simazine: 2-chloro 4, 6-bis (ethylamino) -s-triazine) simultaneously. Results are reported from several field trials in industrial cooling-water systems using the same feeding equipment as for Betz Slimicides C-77P (tableted BCDMH) and C-78P (granular BCDMH). The combination products are highly effective, easy to use, and safe. Results of the field trials establish efficacy of the treatment and define relevant application parameters. A guide for successful application is provided.