Oyster and mussel shells have been used in previous studies as filtration media to remove contaminants, such as metals and nutrients, from stormwater and wastewater. An additional benefit of oyster and mussel shells is they increase hardness, which reduces the toxicity of metals present. This study attempted a practical application of this technique and was undertaken to evaluate whether oyster shell retrofits placed within the footprint of existing stormwater catch basins could remove these contaminants and increase hardness. Additionally, it was important to determine whether this non-proprietary technology could be evaluated through the Technology Assessment Protocol—Ecology (TAPE) program (Ecology 2011). If this technique could be evaluated using TAPE protocols, then future municipal stormwater managers could possibly use this as a stormwater management tool under a regulatory framework.