The shell weights of planktonic foraminifera can be used as a proxy to reconstruct seawater [CO32–] in the past and, therefore, to provide clues regarding the ocean carbon cycle. Foraminiferal shells must be cleaned prior to weight measurements in order to remove detritus trapped within the shell chambers. The standard cleaning procedure, which involves an eight-second sonication step in water, causes greater breakage of thin-walled shells relative to thick-walled shells, thereby biasing weight measurements. Furthermore, the extent of shell breakage is greater for samples that have undergone significant in-situ partial dissolution. Because the sonic cleaning efficiency of sodium hexametaphosphate is much greater than that of water, we recommend a cleaning procedure based on a four-second sonication step in a 2% sodium hexametaphosphate solution. This procedural change provides a greater detritus cleaning effect and minimizes shell breakage, making shell weight measurements more reliable.