I am a new Betta owner

and need some help.
My 'Sparkle' is in a 5.5 gallon tank with 2 silk plants, filter, and 'luster gems' as substrate (I do not particularly like gravel). He told me he is very happy in his big home ... lots of room to swim around + filter is not too 'pushy'.
But my question is: If I do a % water change every week, how do I get the 'poop' and food he dropped from out of the substrate and bottom of the tank?
I was doing a COMPLETE water and cleaning change once a week, but I understand that certain 'things' (I forget what they are called) are supposed to 'grow' on tank walls and stuff to provide acceptable tank water quality. I want to switch to the %-water-change method, but I am concerned about the stuff that drops to the bottom.
I keep HIS home cleaner than mine.
Another thing I noticed and don't know if it is 'normal' ...
I know this might sound stupid because he IS a FISH, but my tank and plants have a strange "fishy" odor. Even the filter has it. BTW...I have changed the filter 2X in the last 45 days.
Sorry for going on and on ... but any help is appreciated. I want my Betta to be as happy (oh yeah...and spoiled) as my 7 Kitties (they love watching him swim around).