Freshwater is a very diverse set of habitats located across the world, given this there is a wide range of different mineral contents within the waters fish inhabit. This mineral content will also vary in elements and compounds present.

Minerals are important for living things to function: Sodium (Na) and calcium (Ca) for example is used in the nervous system and; Ca and Magnesium (Mg) is important in bone growth and repair.

For fishes there might be multiple sources of minerals whether it be dietary or environmental. For Ca around 50-97% depending on the species is taken up from the water itself (Baldisserotto et al., 2019; McCormick et al., 1992; Liao et al., 2007). As far as suggested in the literature fishes are able to adapt to higher and lower levels (Sanderson et al., 2021) although there seems to be no research in how freshwater fishes adapt to none or extremely low levels.

Mg is similarly around 50% is taken up from the water itself in freshwater fishes (Baldisserotto et al., 2019). This value does not seem to vary in Tilapia as to how much is taken up from the water where there is higher or lower dietary additions (Van der Velden et al., 1991) although in goldfishes where dietary Mg was low the fishes has a significant increase in magnesium uptake from the water (Dabrowska et al., 1991). This shows that the amount of magnesium required in the water is at least varies depending on the species but given not every species will be studied it is an important element to provide both in the diet and water.

These are just two examples of the many elements vital to life that maybe need to be considered by a fishkeeper. The only issue is we can’t often test for them so it’s best maybe to work on assumptions, water changes will definitely replenish these elements and many reverse osmosis remineralisation powders/salts contain these.

The influence of pH

The reason I never went into detail regarding Na and Cl is because there is a bit more to it then whether these are present in the water or not. pH has a strong influence on the whether fishes can uptake or maintain levels of these elements. At a low pH of 5 uptake is inhibit and Na loss is increased, higher calcium levels can be important for preventing this. Interestingly fishes some fishes have adapted methods to handle this e.g. neon tetras and angelfishes (Eddy & Handy, 2012).

Conclusion

This is just an example for the importance of water changes in not just removing waste but replenishing mineral content. While we don’t really know the exact levels required for all the fishes we keep we can make assumptions based on the conductivity or TDS based on where fishes are located in the wild.

References

Eddy, B., & Handy, R. D. (2012). Ecological and environmental physiology of fishes (Vol. 4). Oxford University Press.

Baldisserotto, B., Urbinati, E. C., & Cyrino, J. E. P. (Eds.). (2019). Biology and physiology of freshwater neotropical fish. Academic Press.

Dabrowska, H., Meyer-Burgdorff, K. H., & Gunther, K. D. (1991). Magnesium status in freshwater fish, common carp (Cyprinus carpio, L.) and the dietary protein-magnesium interaction. Fish Physiology and Biochemistry, 9, 165-172.

Liao, B. K., Deng, A. N., Chen, S. C., Chou, M. Y., & Hwang, P. P. (2007). Expression and water calcium dependence of calcium transporter isoforms in zebrafish gill mitochondrion-rich cells. BMC genomics, 8(1), 1-13.

McCormick, S. D., Hasegawa, S., & Hirano, T. (1992). Calcium uptake in the skin of a freshwater teleost. Proceedings of the National Academy of Sciences, 89(8), 3635-3638.

Sanderson, S., Derry, A. M., & Hendry, A. P. (2021). Phenotypic stability in scalar calcium of freshwater fish across a wide range of aqueous calcium availability in nature. Ecology and Evolution, 11(11), 6053-6065.

Van der Velden, J. A., Kolar, Z. I., & Flik, G. (1991). Intake of magnesium from water by freshwater tilapia fed on a low-Mg diet. Comparative Biochemistry and Physiology Part A: Physiology, 99(1-2), 103-105.