Heater problem

Hey I might of killed a few fish and I'm not sure whats going on. So i bought some tropical fish a few weeks ago. I couldn't afford a heater at the time but the employee said they would be fine with out a heater for a few weeks until we could buy a heater. i have about a 25 gallon tank. Everything was going fine all the fish were great. they seemed a little weird at first but eventually got comfortable with the temp. So yesterday i bought a heater for a 30 gallon tank because the only other one was for a 20 gallon tank and i thought that was too small. So i stuck it in really hoping that everything would be great. i woke up in the night to check on them and 4 of the smallest fish i had died. And the other fish look like a mess. Did i do something wrong wight the heater? Any answered are really appreciated.