While the influence of breeding dogs and cats as pets on the environment has been explored to some extent, the impact of keeping companion fish has remained unexplored - until now. Dogs in particular have a significant carbon " paw print ". The average dog (weighing 10-20 kg) in Europe is responsible for 349 to 1,424 kg of CO₂ equivalent emissions per year – compared to 150 to 251 kg of CO₂ equivalent for an average-sized cat (weighing 2 kg to 6 kg). However, there has never been a study looking at the carbon footprint of domestic fish, despite only the UK owning them 4 million households – 70 % of them are tropical freshwater fish. Mine study found that the carbon footprint of a UK tropical aquarium ranges from 85.3kg to 635.2kg of CO₂ equivalent per year – equivalent to 1.6 % to 12.4 % of the annual emissions of the average UK household. This estimate is based on a range of scenarios including aquarium sizes between 50 and 400 liters and different tank operating conditions. But most home aquariums sold by retailers are in the 50 to 100 liter range. ( William Perry, Cardiff University)