Water chemistry varies greatly across zebrafish culture facilities, with no guidelines or even basic advice available on many parameters important for healthy populations. This is very surprising as we have known for decades that the many aspects of freshwater chemistry have profound and varied influences on the physiology and health of fish. Hardness ions (especially calcium) control gill and skin permeability and epithelial integrity. Sodium and chloride are the major ions that freshwater fish must actively take up from the water for survival and are also vital counter-exchange ions required for regulating internal acid-base balance in response to the challenges of daily life (feeding, muscular activity, handling stress, high CO2 etc.). We are also ignorant concerning the health impacts of elevated CO2 on zebrafish. Climate change research has recently demonstrated a wide range of detrimental impacts of relatively small elevations in CO2 in aquatic animals. Many of these have clear welfare implications (e.g. reduced ability or even inability to respond to positive or negative odours, sounds, and visual stimuli, impaired learning, increased anxiety, and also reduced immune response and resistance to disease). At the same time recirculating aquarium systems (as used for zebrafish culture) have notoriously high CO2 levels, but this can be improved quite cheaply with effort and appropriate technical knowledge.
There are large differences in the approaches used to building new or refurbishing old zebrafish facilities, as well as in the resources available, husbandry experience of facility managers and the 'drivers' of research within each facility. This likely contributes to the lack of standardisation/guidelines for water chemistry in zebrafish culture. However, it is vital to determine ideal ranges of the major water parameters for zebrafish culture. This will directly aid in minimising mortalities, wastage, and improving production of higher quality embryos.