This showed up in NY when they mandated the switch from 5 gal to 3.8 gal tanks and as a landlord who now must pay for the water, you had a choice of a couple of bricks in the tank to reduce volume or this type gizmo...

Having seen my share of cracked 5 gal tanks due to idjits dropping the bricks back in after cleaning the tank, I can see this as an alternative but there is another gizmo which is a PVC type dam you install to reduce the volume...less mechs to go wonky.