So i finally upgraded Cosmo to a 5 gallon hexagon tank! He LOVES it and doesn't know what to do with himself.

My question is this: my old 1.5 gallon never really got below 76 degrees, even without a heater. The thermometer on the new tank says 72 so I went out and got a little heater to warm it up. Can I just put the heater in there with Cosmo in and turn it on? I guess I was just worried about the electric aspect of it. The heater is for a 2-5 gallon tank so I'm not worried about it getting too hot, I just throught maybe I should wait and install the heater during a partial water change. Any suggestions? Sorry this was kind of a long post