The use of rare earth elements has increased in recent years, leading to a rise in environmental concentrations. Despite the growth in number of studies regarding toxicity, knowledge gaps remain. For Daphnia magna, standardized test methods involve exposure periods of either 48 h or 21 days to assess toxicological effects. In this study, the exposure period was adjusted to 7 days to evaluate sublethal endpoints not measurable in 48-h tests. Additionally, this approach enabled us to obtain results within a shorter time frame than that required for 21-day tests. This study focused on the individual toxicity of lanthanum (La) and gadolinium (Gd) to Daphnia magna. We assessed mortality, feeding rate, somatic growth, and maturity under static conditions, modifying the media by adding MOPS buffer to maintain an initial pH of 6.8 and providing Raphidocelis subcapitata as a daily food source. Results showed that the solubility of La decreased considerably, with the highest recovery rate dropping from 133.33% at the start to 32.73% by the end of the 7-day exposure period. In contrast, Gd solubility remained stable, with a recovery rate of 86.88% at the start and 81.30% at the end of the test. Daily lethal concentrations (LC