Overflow durso pipe size

[Fat_Fender]

I have a 80L (21gallon) tank and a 15/20L (5 gallon) sump.

I'm making a durso drain for it and I'm unsure what size draining I need

My pump can go as low as 400L up to 1500L/ph. (100-390gallons)

I've read that 1/2" can do 420gph (1500L)
also a return pipe size

Many Thanks

 

[Crustaceon]

1/2” because by the time the water from the return pump reaches the tank, that flow is going to go from 390gph to only around 100gph, if not less.

 

[Fat_Fender]

Should I look at upgrading my pump then, this make my flow only x5 the tank size. If I went to 800pgh would the 1/2" piping still be fine?
Also with the pipe sizing is it the outer size or inner size they are talking about.