Sunlight is mandatory if you want good colour developement for your ranchu. One point to note is that it will be good for you to provide some kind of shaded portion in your tubs or pond where the ranchu can take cover from the blistering heat. UV-rays gives your ranchu the colour whereas infra-red rays gives the heat. You should go for the UV-rays and not the Infra-red rays.

Putting a shade over the tub/pond will prevent lot of the heat from boiling your ranchu.