Answered by Shaykh Faraz Rabbani
Is the water from the tank in which my goldfish swam religiously impure given that my goldfish ejected their bodily waste into it?
Answer:
Walaikum assalam,
Imam Ibn Nujaym affirmed in his al-Ashbah wa’l Nadha’ir:
“And excrement is filthy (najis).”
Imam al-Hamawi discussed the issue of fish excrement, concluding that,
“Fish excrement is pure (tahir).” [Ghamz `Uyun al-Basa’ir Hashiyat al-Ashbah wa’l Nadha’ir, 2: 15]
And Allah knows best.
Wassalam,
Faraz Rabbani.