Copper-based herbicides are commonly used to treat harmful algal blooms (HABs) as they are more effective and affordable than other treatment options. A limitation of copper use is that it rapidly complexes with organic material in aquaculture ponds and rapidly precipitates into the sediment. Over time, copper can accumulate in sediment and contaminate pond systems. Strategies are being developed to reduce the amount of copper used to treat HABs. It has been hypothesized that applying a combination of copper and organic herbicide may be more effective in treating HABs than using copper alone. The aim of the present study was to assess the efficacy of different organic herbicides mixed with copper towards cyanobacteria. Two algal genera/species (Anabaena spp. and Microcystis aeruginosa), which commonly bloom in aquaculture ponds, were exposed to binary mixtures of copper and an organic herbicide (bispyribac sodium, diquat, diuron, and penoxsulam) using dose-response assays. Mixture experiments followed a radial mixture design, with seven independent experiments using fixed ratios of copper to organic herbicide (100:0%, 87.5:12.5%, 75:25%, 50:50%, 25:75%, 12.5:87.5%, and 0:100%) at eight treatments per experiment. Experiments were conducted in 96-well plates, with chlorophyll and phycocyanin intensity measured daily. Results indicated that copper mixed with bispyribac sodium, diuron, or penoxsulam had either additive or antagonistic effects. Surprisingly, Anabaena were more sensitive to copper/diquat mixtures, with a ratio of 75:25 being the most effective, aligning with the Southern Regional Aquaculture Center’s recommendations on diquat use. While promising, the study mainly investigated organic-inorganic mixtures aimed at reducing copper usage for HAB treatment. Further research is needed to explore whether other organic herbicides, when mixed together in binary or more complex combinations (tertiary or quaternary), could be more effective than copper in controlling HABs.