The affix aṇ (a) is added to the word visārin when a word expressing 'fish' is to be made.,
Thus वैसारिणः \a fish\. Why do we say \when it means a fish\? Observe विसारी देवदत्तः \Devadatta who is gliding out\.,
विसारिणो मत्स्ये
Adhyāya 5 · Pāda 4 · Rule 16