✅Answer: Please pay attention to the following narrations:

1 عَنِ الْعَبَّاسِ بْنِ عَامِرٍ عَنْ أَبَانٍ عَنْ بَعْضِ أَصْحَابِهِ عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: قُلْتُ رَجُلٌ إِصطاد سَمَكَةً فَوَجَدَ فِي جَوْفِهَا سَمَكَةٌ قَالَ يُؤْكَلَانِ جَمِيعاً.

📚Al-Kafi, Vol. 6, p. 218

A person said: I said to Imam Sadiq (peace be upon him): A person catches a fish and finds a fish in its stomach!

The Imam said: Both can be eaten!

2 عَنْ عَلِيِّ بْنِ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ النَّوْفَلِيِّ عَنِ السَّكُونِيِّ عَنْ أَبِي عَبْدِ اللَّهِ أَنَّ عَلِيّاً سُئِلَ عَنْ سَمَكَةٍ شُقَّ بَطْنُهَا فَوُجِدَ فِيهَا سَمَكَةٌ فَقَالَ كُلْهُمَا جَمِيعاً.

📚Al-Kafi, Vol. 6, p. 218

Imam Sadiq (peace be upon him) said: Imam Ali (peace be upon him) was asked about a fish that is cut open and a fish is found inside; so the Imam said: Eat both of them!

✅According to the two narrations above, if another fish is found in the stomach of a halal (permissible) fish, both are halal and can be eaten.

Of course, according to the general narrations, it is necessary that that fish also has scales and is halal.

✅According to the two narrations above, it does not matter whether we caught the fish ourselves or bought it or obtained it through other permissible means.

اللَّهُمَّ فَصَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْرَ