The refugium will be for a bow front frag rack tank im setting up.

Im going to build a refugium out of an aquarium, I originally wanted to use a 55 gallon but the tank is way to tall and about a foot too long. So im going to check out some stores to see if i find a tank that might better suit me.

The only design i can think of is the same set up I have for my 55 gallon at work.
An intake chamber full of live rock, a fuge chamber, then a chamber with the return pump.

Can someone re-educate me on some of the main/new componets needed to have an efficient refugium
Whats the purpose of a deep sand bed?
How many inches is considered deep and does the foot print size matter any?
Why use rubble rock instead of bio balls?
Is chaeto still one of the best algaes to have in a fuge?
What does it take to go skimmerless?
WHat a bubble trap and what does it do?