I have a 5 gallon aquarium with my betta in it and it has a heater, but I don't think it's doing such a great job heating up all the water. My betta spends a lot of time just hunkered down under the heater. Is that normal? The heater I have is from Wal-Mart and is made for 2-15 gallons. I have it in the corner...should i put it towards the middle? I just feel like maybe it's not heating all of the water evenly. Is there a better kind of heater that I should get? Thought there might be a better one...let me know what you think! Thanks!