In acidified water cationic aluminium-species (Ali) are toxic towards fish, due to deposition and clogging on gills causing ionregulation and/or respiratory problems. We wanted to study if sodium silica could counteract Al toxicity both in Al-rich freshwaters and in estuaries receiving acid and limed water. Utilizing in situ Al fractionation techniques and exposure of fish in tanks, the change in Al toxicity following the addition of sodium silicate was studied. By adding sodium silicate to acid Al rich water, the retention of Al-species (Ali) in Chelex 100 resin as well as the Al accumulation on fish gills decreased. Upon mixing with seawater, the Ali concentration in water and the concentration of accumulated Al on gills were lower than in corresponding estuarine mixtures without excess silicates. However, addition of sodium silicate to limed Al-rich river water increasing pH >7, resulted in increased accumulation of Al on gills. Thus, the use of sodium silicate to counteract Al toxicity towards fish might be limited to acid freshwaters.