Flow recommendations are as follows. Note that these figures reflect the actual flow delivered to the screen after all head losses.
For the L2, you want 210 GPH +/- 20 GPH, roughly.
For the L4 and larger, you want 420 GPH +/- 30 GPH, roughly.
Initially, a lower flow rate will help to keep the unit quiet and keep flow even across the screen. As the screen matures, you can increase the flow if needed. Therefore, you have quite a range of pumps that will work.
If you are looking for the pump that best fits your setup and preferences, start by finding a head-loss chart. Calculate the vertical head (distance from the top of the water in which the pump is operating to the centerline of the slot pipe) and then add 24″. 24″ represents a rough approximation head-loss associated with the slot pipe and any elbows and plumbing parts along the way. Locate the point on the curve that represents this head level to determine if the pump you are considering will meet your needs.
I provide a Two Little Fishies 3/4″ Ball Valve with every scrubber so that you may dial back the flow if needed.
My basis of recommendation is the Rio Plus UL pump series (which is unfortunately, no longer reliably available)
Their head loss chart is located on the Specification tab on their website
In general, here are my recommendations:
L2 with standard vertical head (say, ~12″): Rio 1100
L2 with high vertical head (24″ vertical): Rio 1400
L4 with standard vertical head (also ~12″): Rio 1700
L4 with high vertical head: Rio 2100
L8 with standard vertical head (~16″): Rio 2100
L8 with high vertical head: Rio 2500
L16 with standard vertical head (~24″): Rio 2100 or 2500
L16 with high vertical head: Rio 2500
L16 with very high vertical head: Rio 3100