sounds like you have the beginnings of a cyano problem. how often do you clean the cannister? if used like a cannister filter it needs to be cleaned daily. if not than you are just feeding the nutrient levels in the tank. i suggest getting rid of the cannister completely. do you have a
protein skimmer? if not i would look into getting one. skimmers remove nutreints immediately from the water column. cannister filters keep the nutrients in the water column. if allowed to rot than the nutrients are not even removed when you clean the cannister. this is why it is important to clean a cannister filter as often as possible. daily is best.
the bubbles are prolly caused by cyanobacteria. as it breaks down nutrients it releases gasses into the water column. as the day goes on the more cyano there is and the more bubbles there are.
you need to start siphoning out detritus from the tank whenever you do water changes. any of the grey/brown floaty stuff that is around the LR is what you need to remove. sounds like you may also need to siphon the top layer of your sand to pull as many nutrients out of it as possible.
G~
