Fish Tank Heater (Bradley Fold BL3)

Hi everyone I’m looking for a fish tank heater and not fussed about what sort or brand, it’s for a project that I’m looking at a small water heater for about 15 - 20 litres of water. I’m not fussed if it needs a small bit of fettling and can collect, I’m based in Little Lever, Bolton, many thanks for your time. Phil