15 Chilis is a lot to add to a 5 gallon all at once. I'm not saying an established 5 gallon with lots of BB couldn't handle 15, but adding that many to a newly cycled tank is a lot for a tank that small. My 5 gallon has 5 Chilis and a betta, one nerite, one oto, countless "pest" snails and a slew of red cherry shrimp.
My tank is overstocked, technically, but with a massive plant load, weekly 50% water change, good aeration, and extra filter media - i never see ammonia in any tests. And always zero nitrite. I get lots of nitrate both from the bio load and ei ferts.
I would suggest doing a couple 20% water changes through the week, like daily to keep the ammonia in check and read up on fish in cycling. I believe your tank was cycled, but there just wasn't quite enough bacteria to handle a big bio load so soon.
My tank journal.
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