No idea - I'm not that experienced, but from the other threads, I can give you a head-start on the best stuff you can post here, so the others might have a bit more of a clue.
Do you do regular tests + if so, how are the water perameters? Do you monitor temperature + if so, has that been fluctuating + what temp. is it at? What substrate do you use, if any (you said all they've been in with is some daphnia, but I wasn't sure if you meant completely bare tank, or other living creatures) + what decor do you have, if any?
How long have you had them?
Do you add dechlorinator to the water when you do water changes + how much water do you change at aa time + how often?
Do you have any photos you can add of the set up + as harsh as it may seem, the little guy who has passed on?
Although it's a lot to ask, the more information you can give, the better the experienced members here can answer you :happy:
Sorry for your loss too