Ruling on purifying oneself with water that has changed due to rust
Question

We have a problem in our house, which is that the taste of the water that comes out of most of the taps has changed slightly, and even its colour has changed slightly. I do not know for sure why that is, but it seems that the pipes that bring the water to the taps have become rusty, and it is difficult to change them. I know that it is not permissible to use water for purifying oneself if its colour or taste has changed due to some impurity (najasah), but I do not know, if the pipes have become rusty and that has mixed with the water, whether the water becomes impure (najis) or not. Please note that I sometimes drink this water if I am unable to go to the garden tap.
Praise be to Allah.The basic principle is that water is pure (tahir) unless its colour, taste or smell change as a result of some impurity falling into it.
But if the water (or its colour, taste or smell) changes because of something that is pure, such as rust, then it remains pure so long as it may still be called water.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
With regard to the issue of water undergoing some changes, small or great, with the addition of pure substances such as potash (lye), soap, lotus leaves, dust, dough and other things which may change water, such as if there is a vessel of water in which there are traces of lotus leaves, then more water is added to it, so it changes as a result but is still called water, there are two well-known scholarly views concerning that. The first view is that it is not permissible to purify oneself with it, as is the view of Malik and ash-Shafa‘i, and of Ahmad according to one of the two views narrated from him, because this is not regular water, so it is not included in the words of Allah (interpretation of the meaning): {… and [you] do not find water …} [al-Ma’idah 5:6].
Furthermore, those who held this view made an exception for some types of changed water, some of which they were unanimously agreed on, and others concerning which they differed. So if the change (or difference) in the water is part of its nature with which it was created, or it is due to something that it is too difficult to protect water from, then it is a means of purification according to their consensus. Regarding that which has changed with the addition of oils, camphor and the like, there are two well-known opinions. Regarding that which is slightly changed, then as to whether it will be overlooked or not, and whether there is a difference between a change in smell and a change in other characteristics, there are three opinions.
The second view is that there is no difference between a change (or difference) in the water that is part of its nature with which it was created and other types, or between that which it is too difficult to protect water from and that which is not too difficult to protect it from. So long as it is called water and the other substance has not overwhelmed it, then it may be used as a means of purification, as is the view of Abu Hanifah and of Ahmad according to the other view narrated from him, and this is what he stated in most of his answers.
This view is the correct one. "(Majmu‘ al-Fatawa  21/24-25).
Shaykh as-Sa‘di (may Allah have mercy on him) said:
Any water that comes down from heaven or springs forth from the earth is a means of purification, and may be used to purify oneself from major or minor impurity and from filth, even if its colour, taste or smell has been changed with the addition of something pure, as the Prophet (blessings and peace of Allah be upon him) said: “Water is pure and is a means of purification, and nothing makes it impure.” Narrated by the authors of as-Sunan; it is sahih.
Therefore if one of its characteristics is changed with the addition of some impure substance, then it becomes impure and must be avoided."(Manhaj as-Salikin  1/33).
Shaykh Ibn Baz (may Allah have mercy on him) said:
If the water changes with the addition of impurities, then it becomes an impure substance according to scholarly consensus. But if it changes with the addition of other, pure substances such as dye or traces of nearby dyeing and the like, or what falls into water of grass, dust and the like, then it does not become impure as a result, and that does not affect its purifying nature. Rather it remains as it is, pure and a means of purification, so long as it is still called water. (Majmu‘ Fatawa Ibn Baz 29/7).
So if water is changed by rust and the like, that is not a change that is caused by an impure substance, because rust is not an impure substance. So the water is still pure and a means of purification, and the change does not affect you.
But we advise you not to drink this water, because that may lead to problems and be detrimental to your health.
And Allah knows best.