Hello again! I hate to sound like a broken record, but your issue is your water- 24l is only 6gallons. They are being sickened & poisoned by toxic levels of ammonia and nitrite. None of these guys realistically belong in this tank. They need atleast a 40g tank and you need a test kit for your water for ammonia, nitrite, nitrate & ph to monitor these levels.
Your only option is to change most of their water daily with temperature-matched, properly conditioned water until you can upgrade their home to a 40g. If you are not able do this, I would suggest returning them or rehoming them as soon as possible. Heres the fish-in cycling link for you to read again.