Welcome to AA!! Ok, let's fix this!!
4 years ago I had no choice but to do a fish-in cycle because my daughter came home from school with a goldfish in a bag that she had "won" on the last day of school. I did not have a tank at that time, so we went to store bought a little 5 gal
tank for this tiny goldfish. I did many things to help that tiny fish an 4 years later he is a giant, happy 8 1/2 inch fantail goldfish. You can get there too!!
The bad part about putting fish in during a cycle is that there is no beneficial bacteria to use the ammonia yet, so the ammonia will build up in the water and burn the fishes gills and skin. Those burns can get infected and the fish can die. then the ammonia-eating bacteria arrive and start to eat the ammonia and turn that into nitrite, which when high can cut down on the oxygen and basically suffocate your fish to death. None of this so far is sounding too good, right? We'll get to the better part in a sec. Then finally the nitrite turns to nitrate, which, if kept under 40, won't kill your fish. Problem is this all takes at least a couple of weeks.
So, let's fix the problem: you are going to need to Check the ammonia, nitrite, an nitrate levels of the tank EVERY day. Keep ammonia 0.25 or less- anything above that, you will need to do a water change. If it's way above that, you'll need to do a BIG water change. Also buy some Stabilty and Prime and add to tank every day (after water change if you do one). Ammonia will start to drop after about a week or so and nitrites will rise. Again, try to keep nitrites below 0.1 or do water change. It'll take about another week or more for nitrites to reach zero. I've read that adding an air stone during the nitrite period can help add air to the tank, since the fish are getting less oxygen in their lungs. When I had to do this, I was doing a water change EVERY day for weeks. It's a lot of work, but necessary. Then your tank has cycled and you'll need to monitor all levels every few days. Ammonia should stay at 0, nitrites at 0, and keep nitrates less than 20 once your tank is cycled. If done properly, your fish have survived this process.
then you just need to do a PWC
every couple of weeks and your good!!
Just on a side note, they're called starter fish cuz they're cheap and people don't care if they die, but that seems like a horrible way to die: burning all your skin and getting infections from that and then being suffocated after that. No one would think to do that to a dog or cat. Fish are just little harmless, helpless, living things after all. Good luck with your cycle and feel free to ask for help here!! Hope this helps you!!