you can check your tank water temperature with a thermometer, to check your water temp during the day. 64-82 degrees is what they like/tolerate. I keep my freshwater tank somewhere around 76 degrees. I am not sure how hot your water would get, or would need to be before they killed the fish. I actually think they might have died more related to the youth of the tank. However, it could have been a combination of heat/no cycling that did them in. Is your tank near a window?
Planted tanks are really nice, the plants will also decrease any algae growth in your tank. You can actually order some online, or try some other pet shops near you for more stock. Besides that, fish love the plants!
Have you considered putting a betta, and an oto in your tank? Betta's are beautiful, and would absolutely THRIVE in a planted 10 gallon tank. Their colors get nice and bright when they are in a roomy, happy tank. And the Oto will be the cleanup crew
Just something else to consider...