This is covered in Chapter 14 of the iAVs Handbook.
Ideally, it goes from end to end. If your biofilter is 1m wide, and it sits over the fish tank, which is also 1m wide (or more) than your slit drain would be 1m wide. The gap is 5 to 10mm.
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The gap at the bottom of the drain end’s retaining wall could 4 to 10 mm high BUT only a knife cut (edge to edge) is needed in the liner itself. Place strip of shade cloth/non-metal screen and/or small amount of pea gravel covering the slit on the inside to help retain sand.
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