Shastam

Sorry if this has been covered before, but since the weather has been warming up in my area, I've been having a lot of problems keeping my goldfish tank temps low. I had the heater set to 66, but I took it out cause I was worried that may have been the problem. Its not, the water temp keeps getting up to about 76, and I know that is way too hot for my fish. They aren't gasping for air, but they are a bit less active than they were before. Has anyone had problems with this, and how did you fix it? I want to find a way to get the temp under control before summer really gets here!