betta heater

Hi, I have a 13litre tank / roughly 3 gallons and i was wondering which heater to use for it, or wattage, as i have heard stories about cooked fish! Im in the UK if you want to send any links or shop advices! Thanks! :-)

Hi guys, I'm really excited about upgrading my betta's bowl to a 10 gallon tank (currently cycling) I bought a heater at petsmart today but i don't know if i got the right one :-( It's non adjustable but it says it's good for 5-10 gallon tanks, it's a 15 watt aquarium heater, will that be enough...