I did a brew on Saturday and cooled, got in the fermenting bucket and added yeast, added starsan to the chamber in the double bubble airlock and got it under the stairs. Ever since then, the starsan gets sucked into the fermenter, fairly quickly as well (after I replace it). I've heard this is due to temperature change creating a vacuum. I've also heard that that shouldn't happen with this type of airlock. Its not happened before to this extent (i.e. I've never had to replace the starsan).
Any ideas why this is happening? And is there a way I can prevent it? I'm not too concerned right now as the yeast is still working and producing carbon dioxide but when it finishes I still plan to leave it (three weeks in total) so I don't want the possibility of oxygen getting in.