Beneficial bacteria, as a general rule are "not "planktonic, they are sessile, meaning they live attached (as a biofilm) to objects like media, sides of tanks, rocks etc etc.By the way ,turn off your U-V if you half to add benifical bacteria
A UV sterilizer only kills that, which flows thru it.
Of course by changing things the OP may have loosened biofilm, but this is not the norm for beneficial bacteria.

