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?
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.