Hi bro,
Technically speaking 20mm pipe may not handle 3000l/hr of flow.
http://www.reefcentral.com/calc/drain.php?...93&drain=Submit
Based on the below you, after conversion into metric measurement, a 20mm pipe will be able to drain about 1500l/hr.
http://www.reefcentral.com/calc/drain.php?...00&drain=Submit
One of my previous tank with similiar durso pipe, I used Eheim 1048 which have a flow of 600. I never tried to increase to 1500l/hr. But I have tried with a 2000l/hr pump and it definitely cannot handle.
Btw, most reefer with prefer to have 10x the tank volume of flow. Thus for your case, you may need a minimum flow of 3000+l/hr.
HTH