The gill parasitizing monogeneans Pseudodactylogyrus anguillae and P. bini are pathogens which represent a major threat to eel farming. Legislation and selection of anthelmintic resistance make parasite eradication using conventional anthelmintics difficult and emphasize the needs for development of alternative control methods. Conditioning of fish tank water with aluminium chloride (up to 25 mg/l) have been demonstrated to have a negative impact on the infection level of P. anguillae. Zinc chloride (up to 25 mg/1) has a weaker effect. However, the impact of these metals on the parasites is pH-dependent. Lowering of pH to below 6 increases the effect of both Al and Zn whereas the effect of these substances at pH 7.5 is insignificant. Addition of small amounts of especially aluminium to recirculated eel farm water may prove to be an interesting way to prevent pseudodactylogyrosis. The environmental importance of these findings are discussed.