Mitigation of fish-killing algal toxins by clay minerals offers great promise as an emergency strategy for fish farms threatened by harmful algal blooms, but its efficiency is highly clay and algal species (i.e. ichthyotoxin) specific. We here screened several different clay types (kaolin, zeolite, Korean loess and six bentonites) for their adsorptive capacity of extracellular Karlodinium veneficum and Karenia mikimotoi ichthyotoxins as quantified with the rainbow trout RTgill-W1 cell line assay. Treatment with Korean loess, zeolite (0–0.5 g L ^− 1), polyaluminium chloride (0–0.1 g L ^− 1) and clays modified with this flocculant (0–0.25 g L ^− 1) could not significantly improve gill cell viability compared to toxic controls. Kaolin only demonstrated effective removal in case of K. mikimotoi, but concentrations required for complete removal of cytotoxicity were at least 2 x those required for bentonite. Bentonites of high swelling capacity and ideally small particle size (<2 µm) proved best suited for ichthyotoxin removal against both algal species (100% removal at concentrations as low as 0.1 g L ^− 1). Complete elimination of K. veneficum and K. mikimotoi toxicity towards the rainbow trout gill cell line was achieved by bentonite clay, demonstrating the potential to control ichthyotoxicity in an aquaculture setting through targeted clay application.