Yep, your heater is not doing its job. Replace it with a Ebo-Jager or a Titanium. The heater will have an internal thermostat that should kick on as needed, and obviously keep the water temp maintained at night when the lights are off and the house temp is low. Be sure to have a large enough one for your tank size, but that shouldn't be very big for a small tank like yours. I have a 125 gallon tank and have a 17", 250 watt Ebo.
Remember, small tanks will change water quality and temp much, much faster than a larger tank. They are quite prone to instability.
You should aim for 1 inch of fish (go by their size at maturity) for every five gallons of tank water. More than that will eventually stress out your fish and give you disease and water quality problems. In other words big headaches and loss for you.
Many people have lamented the loss of a mandarin here and that is why you hear all of the warnings. It can take over a year for them to slowly starve and waste away. The general rule of thumb is 100+ pounds of live rock per mandarin to sustain them. They forage for microfauna (pods) in the tanks live rock for their main source of sustainence. Please consider this and consider the ethics/ motivations of the fish store that sold it to you. They are a very beautiful, challenging fish, and deserve a nice life.
Please continue to visit the board often and ask for advice and help when you need it. It is a thousand times more valuable than anything that you local fish store can provide. Get to know the forum search function here. You can look up previous threads and find some great info. Try a search on 'mandarin' for example.
I recommend purchasing the book 'The Consientious Marine Aquarist'. It will be an invaluable reference and is good reading. It is some of the best $$ I have spent in this hobby.
Good Luck,
-Christine
