Hotlemond is right, Weipro needs longer break-in period.
Besides that, it could be due tuning of the air in-take. What most reefers do with Weipro is to install a valve at the air in-take tube of the venturi for some fine tuning works.
Remember, always expect some form of skimmate even if your tank is brand new, there is not such thing as "my water is too clean".