If some pork falls into water and none of its characteristics are changed
Praise be to Allah
It is not clear to us how the tap water could be contaminated with impurities of pork origin. What appears to be the case is that such a thing cannot really happen; rather it is merely whispers from the Shaytaan to which the questioner has fallen prey.
What the one who is faced with such whispers must do is ignore them and not give his mind free rein to follow them; he should remember Allah, may He be exalted, a great deal, call upon Him and seek refuge with Him from the accursed Shaytaan. If he continues to give free rein to those whispers, the Shaytaan will gain power over him and will corrupt all his affairs, and he will begin to doubt everything around him, and will be in a state of constant anxiety, distress and confusion.
Moreover, we also advise you most emphatically to go to a trustworthy specialist doctor, for compulsive thoughts and whispers are a sickness like any other, and needs medical pharmaceutical treatment as well as behavioural therapy under the supervision of a trustworthy specialist.
For more information, please see the answer to question no. 62839
Secondly: if we assume that what the questioner mentioned is correct, and that there is some kind of impurity (from a pork source or otherwise) that fell into a tank of water, but the water was not changed by that impurity, then the water remains pure and it is permissible to drink it, use it and do wudoo’ with it.
The Messenger (blessings and peace of Allah be upon him) was asked about doing wudoo’ with water from the well of Budaa‘ah even though the flesh of dead dogs and other kinds of impurities and putrid things were thrown into it, and he said: “Water is pure and is not rendered impure by anything.” Narrated by an-Nasaa’i (326);classed as saheeh by al-Albaani in Saheeh al-Jaami‘ (1925).
So long as the water has not been changed by the impurity, then it remains pure.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: This wording is general in meaning and includes both small amounts and large amounts; it is general in meaning and applies to all kinds of impurity and contamination.
End quote from Majmoo‘ al-Fataawa (21/33).
Ibn al-Qayyim (may Allah have mercy on him) said in his commentary on as-Sunan (1/83):
The fact that he did wudoo’ from the well of Budaa‘ah, even though its condition was as they said, indicates that water is not rendered impure by impurity falling into it, so long as it does not change. End quote.
It says in Fataawa al-Lajnah ad-Daa’imah (vol. 1, 5/84):
The basic principle with regard to water is that it is pure, but if its colour, taste or odour is changed by some impurity, then it becomes impure, regardless of whether it is a small or great amount. But if the impurity does not change it, then it remains pure. End quote.
For more information, please see the answer to question no. 224923.
And Allah knows best.