Answered by Sidi Moustafa Elqabbany
What is the difference between indiscernible filth and excused filth? Can excused filth affect water and make it impure?
There are two possible meanings of indiscernible filth. If you mean filth that was originally discernible but later became indiscernible (like a drop of urine that dried without a trace), this is not excused. However, filth that is so slight that it cannot be discerned by average vision is excused.
Al-Taqrirat Al-Sadidah divides filth into four categories with regards to what is excused:
That which is excused on one’s clothing and in water, such as filth that cannot be discerned by eyesight,
That which is excused on one’s clothing but not in water, such as a small amount of blood,
That which is excused in water but not on one’s clothing, such as a dead fly, ant, or the like, and
That which is not excused at all.
The above is a simplification. There are many kinds of excused filth. Each kind has conditions under which it is excused. If these conditions are not met, it is no longer excused.
So, to answer your question, some excused filth makes water impure while other types do not. Feel free to follow up with further questions.
And Allah the Exalted knows best.
Moustafa Mounir Elqabbany