welcome to fshlore!
so first to get you familiarized, when fish poop, it introduces
ammonia into the tank, they also exhale ammonia. ammonia is toxic to fish. next
nitrite naturally develops and eats ammonia, nitrite is toxic. after that,
nitrate comes and eats nitrite, nitrate is a lot less toxic then ammonia and nitrite, and helps an aquarium thrive.
so what you did was not cycle the tank, you just let the filter run. what a cycle is, is when bacteria naturally develops to eat harmful waste. the problem here is that your filter has no bacteria in it, so when the fish poop, and create ammonia and nitrites, there isnt enough nitrate to eat those.
Fish In Nitrogen Cycle Simplified | Aquarium Nitrogen Cycle Forum | 414083 read this. I suggest you do a fish in cycle. limit the food you feed your fish, do
water changes when needed, and have some ammonia and nitrite neutralizing chemicals. the bacteria you need to cycle a tank, are in the fish!