Used to produce a bouquet or peacock pattern.

Following a design published in Diderot Encyclopedia (1789), the comb features 2 sets of offset teeth which intersect each other when the entire comb is moved in a gentle "S" motion along the long axis of the tank. Teeth are solid brass pins.
Tool dimensions here.

Large Tank Comb: 23 inches
Small Tank Comb: 17 7/8 inches