i would put money on it that it is due to stress , i have seen lots of micros die at young age and there are certain things that micros suffer from that causes them to stress and die.
i know you have many tanks and much experiance but when keeping channa you can through the rule book out the window ,every thing that your other fish enjoy your channa do not.
1st major cause of stress in micro and main cause of death is the excessive water changes , 50% every two weeks is too much , far too much . you should have the tank planted so that you do not need to do water changes so often.
another major cause of stress in micros is no vegitation , they like to hang out under floating plants and hide amoung them , even though micros groqw huge this is not something you should overlook on any channa tank.
third thing is the stressed fish in the pic looks as if its been beeten or chassed for a good while , cichlids are the worst fish to keep with micros at juvie size ,the cichlids are far bolder than a juvie micro and will harrass them ,again causing stress
then of course there is the other channa in with them that could also be causing problems,
the best thing you could do is , keep the channas on there own and set the tank up to suit , since you are not testing your water ,that is probably a even better reason to get some plants in there.
if your not going to plant it the next best option is to age your water before you do the change , ideally age 6 weeks but if you only have one container then once every few weeks,
before i got right into snakeheads i lost my fair share of micros and pleuros , and i am certain the vast majority where down to changing water perameters
cheers col