When you add water to tropical aquariums, you have to make sure that the temperature is just right. Remember the tank is for tropical fish which are more inclined to relatively warm temperatures in contrast to the cold water fish. To get the exact kind of temperature that the fish need, inquire from the shop you get the fish from. You need a light for your fish tank which should be about 5 watts. You need to keep the tropical fish tanks as far away from direct sunlight as possible. This is to prevent growth of algae and other nasty bacteria.