Okay that's fine. can you get any Frozen or Freeze-dried Daphnia? This would help get their bowels moving and if anything is in their intestines/stomach then it will flush it out; getting rid of potential parasites. It might be a good thing to at least try since you've already had two deaths. Even if they aren't quite related, it's a good way to keep them healthy too.
Also feeding them mosquito larvae, not sure if you've got lots of mosquito's where you are or if it's too cold? But if you do, you can set out a small bucket of conditioned water, it can be old tank water as well and let mosquito's multiply in there. And in a day or two you'll get their larvae and you can scoop some out, give them a rinse with some new conditioned water and then you can use a dropper or spoon of sorts to feed them a few at a time. You don't want to dump it all in in case they don't eat them, you'll end up with mosquito's in the house annnnnddd that's never fun!