i dont think the core is the problem since making a loop (eheim->core->basin) the flow rate is very high.
[edit]
the problem is the WW. i think it's probably dirty
[edit2]
yep. just as i thought. there was a lot of
gunk from the cardboard i used to protect the core's fins. will the discoloration have any impact on the WW's performance? (the pic makes it to look more gross than it really is)