Your pump should be able to handle 25% of your fish tank volume per hour, at the head height you need.
For example, if you have a 1000L fish tank that is 1.5meters below the top of the biofilter, you will need a pump with a flow rate of 1000L/p/h at a head height of 1.5m.
Additional information can be found in Chapter 6 of ‘An Introduction to iAVs’
Here’s the relevant excerpt:
“3. Water Pump: Choose a pump capable of emptying the tank in an hour at the rated head height. Pump 25 to 30% of tank volume (15-20 min ON) once every 2 hours starting at dawn, with no pumping at night.” This suggests that the pump should be capable of emptying the entire volume of the fish tank within an hour. The pump should be set to operate for 15–20 minutes every 2 hours, starting at dawn, with no pumping activity during the night. The volume of water pumped during each cycle should be 25-30% of the total volume of the fish tank.