Hey Johnson! Thanks for your reply! They are ok now!

Things do happended like they breath heavily at the surface of the water after they feed sometimes, like they really got big problem. I sometimes change 50% of the water if the situation is critical. Anyway, how come they will be like this? and do i really need to change the water everytime they happended to be like this? The water is clear and clean at all time! Please help me out!
Thanks!

Regards,
John