Your tank is probably in a cycle if it is a week old. I am surprised that you do not have any amonia, you may have nitrites which are also not good for the fish. When you set up the tank, did you use a cycled filter and substrate or was everything new? You should get a test kit to check your water parameters, amonia, nitrite, nitrate, PH, etc. This will help to let you know if the water is stable for the fish to live in.
You stated that you added a heater, please do not take offense as none is meant, just trying to help--do you have a filter on the tank?
If the tank is cycling, which it probably is, that is probably the reason your guppy is having problems. The tank has to go through the nitrogen cycle where bacteria grows converting amonia (fish urine) to nitrite, and then other bacteria grows converting nitrite to nitrate. Fish can live with the nitrate in small amounts. Water changes keep the nitrate at a comfortable level.
I would suggest monitoring your water by getting the appropriate test kits and doing daily 20-25% water changes to keep the amonia and nitrites down. It will take longer for your tank to fully cycle, but it will help your fish.
Good luck.