Dandre, do you have a thermometer inside the tank to monitor the temp?
You really should have a heater, and you can get one that self-adjusts so that if it gets a bit colder at night, the heater kicks in to keep the water temp steady, and also for the times of the year it gets colder where you are.
Keep in mind that fish are coldblooded animals - they can't self-adjust their own internal temperatures like humans and other warmblooded mammals can. A couple of degrees in fluctuation can mean life or death for them, paticularly when they are in a small tank environment - and by 'small' I don't mean the size of the tank - any tank is 'small' compared to the natural lakes and streams these fish were genetically adapted for.