Does the water remain pure, and is it permissible to perform ablution (wudu') with it and to cleanse oneself with it from ritual impurity (janabah) if a mouse falls and dies inside the water tank?

If the color, taste, or smell of water changes due to a dead mouse, then it is impure (najis), and purification with it is not valid. However, if it does not change and reaches two qullas (a measure of volume), then it is pure (tahur), and what falls into it does not harm it. If it is less than two qullas, the majority of scholars hold that purification with it is not valid, in contrast to the Malikis who consider it valid as long as none of its characteristics have changed.