That's my logic too (see my other post). But people seem to be using 4" for bigger tubs without problems. I talked to a breeder here the other day and I brought up the 11" for bigger tub he actually advised against it. I was asking about 32 quart sized tubs thought, but still 4 inches doesn't cover 1/3 of that. I also see Ralph Davis videos where his freedom breeders and ARS racks with adult tubs only use 3 inches. It obviously works, but it doesn't make sense to me that people recommend 1/3 and at the same time use 4" for everything hatchling to adult tubs... I don't understand it.