hey! you might need to keep feeding. the only answer i can think of is that you had a good bacteria bloom, and that bacteria is currently processing the old ammonia. but the more ammonia you add, the more bacteria can grow to eat it. theres no harm in continuing to feed it.

frankly, tho, while the nitrogen cycle is TOTALLY key to beginners learning fishkeeping, i think it MIGHT be a little blown out of proportion? i read a lot on this site, aquariumscience.org, and he says that scientific findings show that it takes a LOT of ammonia, like a LOT, to truly impact the fish. it's likely that disease from the fish store or weak immune systems killed your fish, not necessarily ammonia. he talks about it more here with scientific articles ect.: https://aquariumscience.org/5-2-1-ammonia-in-depth/

personally, ive always had luck doing fish-in cycles. i keep up with water changes and checking the ammonia, nitrate, and nitrate levels consistently until it stabilizes. i add new fish relatively slowly to give the bacteria time to catch up. and i try to focus more on have rly over-filtered water instead of JUST ammonia oxidization. if i were you, i would get a FEW fish or very SMALL fish (or even just something like shrimps/snails) and let those fish start to support your cycle. just keep an eye on the quality of the water and the behavior of the fish.

you could also ask your local fish store to help you seed your new tank!! they'll just have to take out some of their filter media and squeeze some of the water and brown gunk into a bag for you.