silver tipped tetras dying

so this is totally weird, and really depressing. my girlfriend and I are usually really good with taking care of our fish, and none of our tanks usually need anymore than the usual maintenance for clean healthy environments. our newest 20 gallon tank has been set up now for a few months, and been stocked lightly with four platys, one gorami, and five silver tipped tetras.

its level are:
nitrates: 18 ppm
nitrites: 3.2 which i know is a little high, but i'm working on it.
totally soft water
0 chlorine
124 alkalinity
6.8 ph

all of these fish seem to have been doing fine, until today. when i woke up this morning, i went in to feed the little buggers and two of the tetras had just sunk to the bottom. everyone else looked healthy and happy. I did a 30 % water change, and tested all the levels. hours go by and both of the tetras have now passed. they looked fuzzy almost, and their fins had seriously deteriorated. they didn't look great this morning, but i've never seen anything happen like this so quickly. either way, now my other tetras are swimming on the bottom, not scratching but acting the way the others did this morning.

please, someone help.