We have a18 litre tank...I need to know do I need a heater in my fish tank. We live in the tropics and the temp is never below 22C degrees inside. On average the temp would be between 22 - 28C in the house. I have an 1 orange Molly and some fry. The orange one is constantly hiding near the pump and only occasionally comes out to feed. If I need a heater what type do I get. Thank you.