I'd like to have enough of a deep sand bed for a yellowhead jawfish in my new 125, but I don't really want to do a DSB for the whole tank. I'm thinking of doing maybe 1/4 of the area as DSB, using live rock as a dam to hold the sand back from the rest of the tank, but I'm not sure 1/4 of the area will be enough. How little area could I have deep and still have the jawfish be comfortable? There will be lots of open sand as the live rock is done sort of "bonsai" style.