The JUWEL Aquarium Heater is designed to fit perfectly inside the filter system freeing up space in the aquarium and keeping it safe from your fish. It is supplied with the heater holder necessary to install it inside the Juwel Aquarium Filter System and also with suckers to attach it to the glass of the aquarium if it is to be used outside the filter. Juwel heaters are available in four wattages: 50w, 100w 200w 300w The heater is automatic, once set to the correct temperature it will heat the aquarium water and once the desired temperature is reached will automatically switch off.