What is the ruling on water that contains ornamental fish — does it become impure on account of their waste?
All praise is due to Allah, and peace and blessings be upon our master the Messenger of Allah ﷺ.
The water in a pond containing ornamental fish is pure so long as it has not been altered, for a small amount of fish waste (rawth) is pardoned and overlooked — provided it does not change the water.
It is stated in Ḥāshiyat al-Jamal ʿalā Sharḥ al-Manhaj (Vol.1/P.178): "A small amount of hair from a non-edible animal is pardoned, as is a small amount of hair from a riding animal — on account of the hardship involved in avoiding it — and likewise a small amount of fish waste is pardoned, unless it alters the water." And Allah the Almighty knows best.
Must a person refrain from eating for the rest of the day if they break a fast of a vow (nadhr) or a make up fast (qada)?
Whoever observes a vowed fast (nadhr) or a makeup fast (qada) is prohibited from breaking it without a valid excuse. If they break it without a legitimate reason, they are sinful.
However, they are not required to refrain from eating for the rest of the day, because such restraint is only required out of respect for the month of Ramadan, not for other types of fasting.
Is it a condition for I‘tikaf to be performed in the mosque?
Yes, for I‘tikaf to be valid, it must be performed in a mosque. I‘tikaf is not valid if performed at home or elsewhere.
Is it permissible to offer condolences to a non-Muslim?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah.
It is permissible to offer condolences (Ta'ziyah) to a non-Muslim upon the death of their relatives. This constitutes a form of benevolent treatment (Birr) that we have been commanded to observe. Allah Almighty states {what means}: "Allah does not forbid you from those who do not fight you because of religion and do not expel you from your homes—from being righteous toward them and acting justly toward them. Indeed, Allah loves those who act justly." (Surah Al-Mumtahinah, 60:8). And Allah Almighty knows best.