I think for best performance you'll want to get a converter that converts from the Eheim's metric threads to english FPT/MPT.
Myself, I go ghetto style and use a whole roll of teflon tape with MPT. But I realize this is bad, and who knows if it causes a reduction in flow.
If you find a metric to FPT adapter, please let me know, I'll buy a ton of them.
Thanks
Josh