For those of us on the old-school measuring system...
1 ha = 2.5 acres,
140 l/min = 5 CFM
I would say, off the cuff without all the specs on your system, that you have a decent sized aeration system for your pond if fish habitat was the main concern. It might be too small if you see summer water temps above 85 -90 degrees F (30-32 degress C). It that case you would want to only run at night and I bet you would need more diffusers and more airflow.
But, you are talking about a swimming pond and that is a whole different animal...Water temp is not as important and aethetics becomes top priority. Any dissolved oxygen you can get to the bottom will help with the nasty muck that accumulates down there. Too much aeration and you can stir up the water and cause it to be muddy looking which deters swimming depending on what you think a good swimming hole looks like.
Do you have fish in it (if so, what kind)?
Are they important to you?
Can fish get into pond naturally (from upstream from other ponds)?
What is the Secchi disk reading for clarity?
What, exactly, is not so conducive to swimming in it now?
It looks beautiful, by the way! I love it, and the scenery too!