Try pellets. Also, if it's a new fish, it might just be adjusting to its new surroundings.
Tanks will be a few degrees cooler then the room temp, and that is being generous. A room kept at 78 will result in say a 2 gallon being between 69-72... too low for a healthy fish. In order to keep the water at the right temp using just room temp the room would have to be closer to the 90s.You 100% need a heater unless your house or room the fish is in is always at 76F.