How did this die???

So i recently bought two baby fire eels one at 3.5 inch and one at 4inch, they were doing fine swimming freely in my tank, no predators big enough to kill it or even want to, then today i come home and find the smaller one dead on the bottom stiff as a bone

they shouldn't be stressed out because there is nothing to stress them out
water is good, temp good
tankmates- senegal bichir 3.5inch, fire eel 4inch, peacock eel(always burrowed and scared all the time 6inch), bala shark 2.5inch, some snails, and a sailfin pleco 4inch? none of these every go after eachother ever in my tank
lots of hiding places
no diseases or anything noticable on it, just layed there stiff as a bone

my other seems fine just chills by my filter

the other used to hang out by my heater, perhaps it got retarded and overheated? my tank is not hot by any means and the heater is on low because its so damn hot where i live now but it would hide behind that recently

i soooo confused
any suggestions on how it died?

like a year used to have a tiretrack in there for sixmonths and that did fine grew like 3-4 inches then sold it and my peacock did fine, i read fire eels are not as hardy as the others but my water parameters fit whatever liveaquaria lists the recommended water parameters at i cant remember

got them both from the same lfs the one that died last saturday and the other one wed and they both ate at the store and were eating the week i had them also they both came from the same tank at the store along with every other kind of spiny eel so there was no previous aggression problems, plus my tank is bigger than the stores

i hope my last one lives otherwise ill be pissed idk why it died its bothering me haha and the one thats living just curls up by my filter now all lonely