I can grow a disgusting biofilm in my water tubing, but not main tank

[beanbag]

My shrimp tank has been cycled and up and running for about 2 months now.  I can quickly grow a disgusting layer of biofilm inside the flexible tubing for my pump and filter system, but somehow not inside the main tank.  I say this based on the facts that:

glass is totally clean

intake tube and output spray bar (both acrylic) totally clean

no algae

I suspect this is the reason why my shrimps have periods of inactivity.  They run out of food for grazing and just camp out.  The exception is the cholla log, where I often see shrimp picking at it.

One thing I sometimes do is give the tubing a few squeezes, and this sprays out a bunch of biofilm flakes, which the shrimp don't really seem to care for.

But anyway I would like to know how I can grow a thin layer of biofilm / algae in my tank.  I assume this is the reason why when I go to an aquarium store, I always see active shrimp grazing.

If I change out my lines with a more inert material, will biofilm grow in the tank instead?

I occasionally dose Bacter AE, which is more like a temporary food than an actual biofilm promoter.