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hi there, welcome to the forum!
First, it is a good idea to have everything set up first before get the fish. In fact, it is also highly recommended to do a fishless cycling after everything was set up first, then get the fish after the fishless cycling is done.
There is insufficient information for me to pin point the problem, but if I have to take a guess, it could be the sudden rise of water temperature. Tropical fish are more fragile and sensitive to any sudden change in the water temperature. If the water temperature go up or go down too quickly (in my experience anything change more than 3C within an hour) could be dangerous for them and even kill them. You should have let the water temperature to rise no more than 1C per hour just to be safe, and make sure there is good water flow in your aquarium to distribute the heat evenly.
You also need to have a separate thermometer to keep eyes on the actual water temperature. Since most of the heaters of the same model will result in different temperature at the same setting, you should not trust the temperature to the setting on the heater itself.
I also hope you have a filter system. It is even more important than the heater if you want to keep fish in a closed system like an aquarium. Without a filter system running 24/7, the fish can die to ammonia poisoning eventually.
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