i had similar issues with baby aba aba knives.. if your not, try surface skimming when doing water changes so when they come up to breathe the thin " layer" is as minimal as possible, i surmised my knives where actually swallowing bubbles and injesting them vs just breathing.. not sure if this is accurate but once i did this i had success with raising one. I just use a wide mouthed cup and skim off about 10% per day, atm doing this for my baby tank as well as a precaution.
this is an incredible thread