Long QT syndrome (LQTS) can lead to ventricular arrhythmia and sudden cardiac death. The most common congenital cause of LQTS is mutations in the channel subunits generating the cardiac potassium current I-Ks. Zebrafish (Danio rerio) have been proposed as a powerful system to model human cardiac diseases due to the similar electrical properties of the zebrafish heart and the human heart. We used high-resolution all-optical electrophysiology on ex vivo zebrafish hearts to assess the effects of I-Ks analogues on the cardiac action potential. We found that chromanol 293B (an I-Ks inhibitor) prolonged the action potential duration (APD) in the presence of E4031 (an I-Kr inhibitor applied to drug-induced LQT2), and to a lesser extent, in the absence of E4031. Moreover, we showed that PUFA analogues slightly shortened the APD of the zebrafish heart. However, PUFA analogues failed to reverse the APD prolongation in drug-induced LQT2. However, a more potent I-Ks activator, ML-277, partially reversed the APD prolongation in drug-induced LQT2 zebrafish hearts. Our results suggest that I-Ks plays a limited role in ventricular repolarizations in the zebrafish heart under resting conditions, although it plays a more important role when the I-Kr is compromised, as if the I-Ks in zebrafish serves as a repolarization reserve as in human hearts. This study shows that potent I-Ks activators can restore the action potential duration in drug-induced LQT2 in the zebrafish heart.