midget...you said moss algae OR duckweed or something else. Can you provide pics so it can be properly identified? If it is duckweed the carp wont provide much help on the duckweed and results are spotty with algae control. If it is duckweed there are a couple of other options besides Sonar RTU if you have constant flowing water. If it is just algae RTU will not kill it. If it is Common Watermeal then Sonar would be one of the recommended chemicals but it might be difficult to control especially with the conditions you have described. Is water running out of the pond all year and if so at about what rate?