New to hobby and board

I have read up on sand beds. For the DT tank you can go with a bare bottom but that may not be as aesthetic as a sand bottom. Also, if you have any gobies that require the sand or sand sifters in your tank you will want sand. Read up on the live stock in your tank, this includes invertebrates, fish, and corals. I personally will not go over 3" of sand in my DT, but in my refugium however I want a DSB about 5 or 6 inches deep. This allows for the correct bacteria that needs to be in an oxygen depleted area, to grow and help eliminate nitrates. Word of caution, once your DSB is established, do not mess with it. It can release harmful gasses into your water column and kill everything.

If I was you, I would research swapping your tank over yourself. Don't pay someone else 400 dollars that you can spend on other things for your system. Moving from a smaller system to a bigger system will require that you need more Live Rock, if you dont already have the 90+ lbs. Also if you are wanting a sand bed you will need another 40-60 lbs of live sand to give you enough coverage on the tank bottom.

You can also use the money you save to buy a RO/DI unit for water changes and top offs, if you don't have one already.

There is a lot more information I and others can give you, but until we know exactly where you are at in this hobby and your dedication you wont get much from it.

 

As little as 3" will can be effective. My bed bubbled all day. There is much debate on properly maintained beds, and not as far as what will happen to them overtime. Everything needs to be maintained, even sand beds. That's what critters are for.