how long can a warm water fish live for in a normal, unfiltered tank??

[hey whatsup]

my bro has a warm water tank. im asking this cuz my brother has this medium size fish that he bought a couple days ago and just yesterday it ate all his fish in his fish tank. HA. thats like 20 fish. and there is 2 things left in his tank, another larger relle fast fish that doesnt stop swimming, and some sucker fish. there were mangled fish bodies everywhere with parts missing, most were gone though. we caught it in the act nibbling the tail of a relle nice fish. i laugh. so he gave it to me and i named it chomper. i just dont know how long its gunna live. i love the fish. its such a bad a*s.
i forgot to sat im taking it back to the fish store either tommorow or the day after tomorow.. im just worried itll die 2night.. OH NO CHOMPER

[Alex P]

Well firstly he didnt eat the "loach"(sucker fish) because its a big fish. and the main question is that i would give him about 2 weeks until he dies i have guppies and they are warm water fish so just get a "tropical adaptor"

[FishRfine]

a day, mabye? most likely less with the decaying body parts in the water.