RBTA: Not so bubbly. Why?

[stupid_naso]

This has always been my experience with RBTA and BTAs, they are not bubbly anymore or not bubbly at all after awhile. Anyone knows the reason for this? I mean scientifically maybe? What can be done to get the bubble back?
stupid_naso

 

[elvis a.]

People have guessed what causes this, studies have been made as well.. We really don't know.. It can be from lighting, water quality, or an amount of other things...