Heater issues

#1
Ive now turned off my heater i got a couple of weeks ago because the temperature keeps getting to hot! Its a topp finn 300 watt for 60 gallons mine is 70 gallons and i have it on low but the light still stays on with a hot temperature. Also I have 2 four foot flourescents for the lid and light that I got at home depot but they dont put off that much heat but I dont know. A couple of weeks ago I had a school of tiger barbs but i was gone for a couple nights and the temo got to 100 my three gouramis were fine Chinese algae eaters were fine tetra fine pleco fine but the barbs suffered a worse fate. Whay might i be doing wrong? The heaters should be really easy right just plug em in and set the dial and check temps?
Thank you for any help