Are you feeding them lab blocks?
Ware Mfg. used to make a lab block hopper, but its been discontinued for some time. Basically its a 1/2" x 1" mesh cage that hangs inside a rodent cage or tank to hold the lab blocks. They have to eat through the mesh and break off pieces of lab block. It prevents them from hoarding all the food and getting it covered in feces, urine and bedding.
You can easily make a home-made lab block hopper with 1/2" hardware cloth. Make a small box out of hardware cloth about 4 x 5 x 7 or so. Size can vary of course depending on the height of tank or tub, etc.
Or use lab rodent cages with wire tops that act as a food hopper and bottle holder.
Those with mouse or rat racks usually have mesh tops on the tubs and just put the food on top of the wire tops. The tubs must be short enough so they can reach the wire top to eat.