I ave 29 g tank, heavly planted, and i have 5 neon black neon and 5 rummy nose, over the last 2 weeks, the black neon will stop eating for few days then it will die, it been happening to one after the other , not to all them at the same time, no other sign of disease to any other fish, even the balck neon, they dont have any sign off sikness other than sopping eating, any suggestion?