Sand filters are pressure filters. If you have an aquaponic system you don't need them, or my bead filters. Those and strictly needed for recirculating systems with no gravel beds. The gravel beds are the filter.

Pressure filters go in the system immediately after the pump, just like in a swimming pool. They must be backflushed routinely to keep the water clear.

Bead filters are non-pressurized. These are placed in a sump and catch the overflow before it gets to the pump that is submerged in the sump. When the beads become clogged, the filter overflows into the sump and no longer filters. It is removed and cleaned.

The scrubby pads unter the bead filter is the bio-filter. In the sand filter system, the aquacube is the primary bio-filter, but the sand filter does have some bacteria and does some bio-filtration.