You should turn off your UV sterilizers. Levamisole is a light-sensitive drug. Turn off lights when you dose. I don't see much literature for Levamisole dosing in fish so I'd guess that's just based on experience. Therapeutic effects can varied depends on fish species as well.
1/4 teaspoon is the equivalence of 1.25mL. Not sure what formulation of Levamisole we are using for fish but the density of Levamisole HCl is 1.31 g/mL. The exact conversion to weight for Levamisole HCl is 1.6375g per 100 gals. You can also do a science experience to figure out its exact density of the levamisole as well. Get a 10mL graduated cylinder and fill it to the 10mL mark with levamisole, then dump it on a sensitive weight scale and measure its weight. Use the weight and divide by 10 to get the density in g/mL. Multiply by 1.25mL to get the conversion to 1/4 teaspoon.