i need the smallest possible heater to put into my tank containing only zebras. i've had the 6 zebra's in there for 4 months so far and on warm days they go to the bottom and cold days to the top of the tank. so i want to put a heater in there so they feel most comfortable.

if you can supply me with links from websites that will b awesome. i live in UK for UK sites will be best if not you can still supply other sites. thanks