it probably does with no sand and rock, but i have about 24 gallons of water in mine with everything in it.. but it will vary depending on how much rock n sand you use
i would recommend mixing up the 29 gallons if your not mixing it in the tank. it never hurts to have extra saltwater mixed for future water changes. like previously mentioned it will depend on the sand bed you put in and how much and the type of live rock you use. some is more porous then other. i usually recommend having at least 10% of your tanks total volume premixed incase you need to do an emergency water change or for some reason you spring a leak.