Hi vatso - DSB = Deep Sand Bed (just the depth of the sand bed that you should use - you can use "Reggies Play Sand" (or perhaps go to Builders Warehouse and ask them for their play-sand).... Between 12 and 15 cm depth is what is called a "DSB".... BUT also important - if you can add a complete (cheaper) tank to act as a sump JUST for the DSB, then your filtering CAN really work well!
Denitrator reactor is a very very long piece of flexible pipe, that gets water pumped into the one side and have "clean" water come out of the other side.... There is an amount of both aerobic as well as an-aerobic bacteria living on the inside of this pipe, and they "clean" the water from nutrients..... (this is very easy to DIY)....