Is the light necessary for the tank? Is it being used for plants or just viewing purposes? If you're not using it for the survival of your plants, there's not really a reason to keep it on all hours of the day. You can turn it on whenever you'd like to look at your axie, but just make sure you have adequate hiding areas where it can get away from the light if need be. As for the chiller question, it depends on the light you have and how large the bulb is. A desk lamp puts off less heat than long fluorescent bulb kept in an enclosed space, such as that of an aquarium hood. You also have to take into account the distance it is from the water; if it's within a few inches, there's a good chance it will interfere with water temperature.