First off I would check the tank and lines then ditch the inline filter, if all is well there then I would move to the reeds and lap the reed block flat and smooth, reassemble and run it and see if it made any difference, a lot of the reed blocks are not flat and don't seal very well. Do one thing at a time starting with the simple stuff and work up from there.
Also make sure the reeds are installed the correct way, they should have a slight arch to them the arch face's away from the reed block, so as you tighten the screws the reeds have some spring tension on them to help seal better.
And no this is not common to have this sort of issues with such a small amount of use.