Well, a highly reactive metal is just that, highly reactive and there's a high chance that it will react with water. For instance sodium, highly reactive metal when placed in a water bath creates a nice explosion (sometimes, depending on how much sodium is used.). So sodium wouldn't be the best choice to make a water tank out of.
I hope that kinda made sense. I don't have anything to back be up besides the fact that I'm doing the same topic and have my exam tomorrow so I hope it's right.
Hope you're enjoying chem.