Question: Magnesium sulphate heptahydrate

[peter5930]

I live in an area with very soft, slightly acidic tap-water that comes from a reservoir in the nearby upland peat moors (60ppm total dissolved solids, negligible KH, PH 6.8) and I've had a high mortality rate when trying to raise axolotl larvae in it, so I've taken to adding various salts to it, according to the instructions given here:

Axolotls - Requirements & Water Conditions in Captivity

I've had much better results since adding the salts, with very few deaths, and the recipe I'm using is the modified Holtrefeter's solution in which MgSO4 is added in the same weight as NaHCO4, so the total composition for the solution at 100% strength is as follows:

NaCl 3.46g/litre
KCl 0.05g/litre
CaCl2 0.1g/litre
NaHCO3 0.2g/litre
MgSO4 0.2g/litre

I dilute this to 30% strength for the axolotl larvae, which brings it down to:

NaCl 1.038g/litre
KCl 0.015g/litre
CaCl2 0.03g/litre
NaHCO3 0.06g/litre
MgSO4 0.06g/litre

However, I'm actually using MgSO4.7H2O rather than annhydrous MgSO4, and I'm not entirely confident about what weight of MgSO4.7H2O I should be using. The molar mass of MgSO4 is 120.366 g/mol while it's 246.47 g/mol for MgSO4.7H2O, so I calculated that I should be using about twice the weight of MgSO4.7H2O as for MgSO4, which would mean 0.4g/litre of MgSO4.7H2O for the 100% strength solution, but I'm not a chemist and I'm hoping that someone will pick my brains and let me know if I've gotten it right.

 

[Jennewt]

Bravo to you for taking the trouble to make the correct solution to amend your water. Your math seems correct to me, although it's been a long time since I did any chemistry.

 

[peter5930]

Thanks; I don't have much experience with managing water chemistry beyond basic dechlorination and filtration, and I didn't want the learning process to involve a tank of dead larvae.