I just got my betta a couple weeks ago, and I got a 2.5 gallon with a light and under gravel heater. I plugged in the heater, there's no thermostat in it, it says it just raises the water temperature a few degrees. I turned on the light and heater and went to work. When I got home the water temperature was up to 90 degrees! The water actually felt like warm water and my poor fish was looking very lethargic and his fins were limp. I unplugged the heater and turned off the light and took off the hood and the water temp quickly went down, but then it was in the low 70's. I noticed If I just leave the light on it keeps the temperature up around 82, which is perfect, but it drops into the 70's if I turn the light off at night. So I've been keeping the light on all the time. Is it ok to do that? Is there any harm or stress caused by not turning off the light? I do have a cave for him to hide in where it's a little darker, but I know it's not the same as having the light off.