All tech and no tank... lolLUL, Magnesium Hydroxide is essentially insoluble in water.
Magnesium Hydroxide solubility:
0.0064 g/1L (25 °C)
For reference
Calcium Hydroxide solubility:
1.73 g/L (20 °C)
Calcium Carbonate solubility:
0.013 g/L (25 °C)
Really the guy in the video doesn't know what he is talking about.