If I understand what you are doing, this can be done using the get date/time function, and entering the on time and off time as a string. You break the string into hour, minute, and second. Then compare the current time to the ON/OFF times to control a boolean indicator. Then you would have to pass out the boolean value to control a switch for a light.
Attached is a working VI that does everything but the actual IO to control the light. Just think of the boolean indicator as being your aquarium light.