All three of my guppies died in one night, but my three ghost shrimp are still living. I have a 25L tank, and the day before I did a 25% water change. I also got a new heater and replaced the old one. I tested the pH and it was about 8.4, but alkaline level was fine. The water was also extremely soft. Could the death of my guppies be caused by an ammonia spike? If so, I was thinking the spike could be caused by the removal of the old heater because when I took it out it felt slimy. Could there have been important bacteria colonies on the heater? What else could have caused it?

The ph level should be 7.0 not 8.5 ( adjust it slowly 0.2 daily. What is your ammonia level?? Why did you change 25% ?
The only time you should change that much water is when there are too many toxins in your tank like ammonia,nitrates, nitrites and so on. Otherwise 10% weekly is advised.Changing the heater did not harm anything.
What is the total amount of fish in this tank?
Remember, never OVER FEED, this will pollute your water fast and never change the water temp. too fast, also deadly.