Blebbistatin, a cell-permeable, highly specific myosin II inhibitor, is widely used to inhibit heart contractions to avoid movement artifacts during optical mapping experiments. Published drug concentrations vary, with 20uM being the most commonly used in zebrafish experiments. However, blebbistatin has a reported poor solubility in water above 10uM, even with 0.1% DMSO in the solution. Our objective was to compare the efficacy of blebbistatin against para-aminoblebbistatin, a new blebbistatin derivative with higher reported solubility in water (400uM). We first sought to determine the minimum concentration of each compound needed to completely inhibit contractions by testing their effect on explanted juvenile zebrafish hearts at 10, 15, and 20uM solutions. We recorded videos of the explanted hearts before and after the application of the drug in five-minute increments. We analyzed the videos using the open-source NIH software Fiji and quantified the movement during contractions at each time point. We then plotted the change in contraction size over time for each concentration of the compounds. We found that for both drugs, 20uM was needed to eliminate contractile activity after 20 minutes of exposure to treatment. However, the presence of solid aggregates indicated a lack of complete blebbistatin solubilization at this concentration. Unlike blebbistatin, para-aminoblebbistatin was completely solubilized at all tested concentrations. Future steps include evaluating the effect of 20uM para-aminoblebbistatin on normal heart function by measuring action potential and calcium transients before and after drug treatment. This result will help optimize the conditions for reproducible optical mapping experiments in the zebrafish heart.