Rana temporaria froglets are in trouble.

[Entoursis]

Good evening.

I have faced a sudden problem recently and hope you can help. As you can understand from the topic name, I'm rising some froglets of rana temporaria. I have been rising them since they have been the spawn, successfully raised them into small froglets. Maybe 2 month already passed since they have lost their tails. For the first time everything was fine, they were growing, eating et cetera. I keep them in 40 cm long tank. Bottom is covered in wet paper and there are some covers. Some ventilation holes are present in the roof and 2 walls, ventilation is average for humid keeping, and walls of the tank usually covered in condensate. Fruit flies are the food. Additional calcium is given by spraying the froglets with water+calcium solution for froglets to absorb it. Temperature is 23 Celsius. This is the general method of keeping these frogs. They have been living for a month under this conditions with no accident. But few weeks ago they started dying away. Very slowly and one by one, which is very strange since in case of bad comdotions they should die faster in large numbers. For the last 3 weeks I'm loosing one froglet per 3-4 days. Nothing really changed in their enviorment since they've been fine. No idea of what is happening.

Please, help, if you can.