(bio) filtering question

i have a 130L Jewel tank which im fairly happy with. it has 3 bala
shark, 4 clown loaches, 2 siamensis, all growing fast, and some
otocinclus. while trying to come to grips with water biochemistry, i
was wondering if its filter allows proper biological filtering?

the filling is made of a plastic sponge type material, including an
active carbon sponge. once a month i do a 50% water change and take
the fillings out and rinse them under the tap. the top layer (cotton
wool type stuff, drenched in waste) and active carbon pad i replace

where would the (good, waste eating) bacteria reside and how many do i
remove by rinsing filter pads under the tap and chucking the top
layer? how can i optimise this process? more/less water changes, get a
new filter or filter medium? should i buy bacteria and add them to the
tank?

the tank is generally ok, ive had all these fish for almost 2 yrs with
none dying, but they are getting big so i realise the importance in
keeping nitrate levels low ... ive gotten some really usefull answers
from this group so im hoping to get the same once again ...

thanks!
stefan