eu,

On the safe side, during your next water change. introduce 0.5% salinity to the water and starve them for 2-3 days and re-start feeding with small amount. After another water change, all routine back to normal. See if it still happens.

My fish had this before and I didn't do anything and yet nothing happens. Donno why....

Hope to hear more from others...

thanks

__________________
Japanese Ranchu is another ball game