Whatr size aerator pump is needed for a 500 gallon aerobic system?
Option 1 - check with the local health department or wherever septic plans are filed locally and see what your system was designed to have - the system design should be on file there from when it was permitted/approved.

Option 2 - hire a septic design engineer. 

Option 1 should be free. Option 2 costs money. Pulling a number out of the hat on the internet is not a good option.